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Document and Entity Information - shares	9 Months Ended
Document and Entity Information - shares	Sep. 30, 2018	Oct. 19, 2018
Document and Entity Information [Abstract]
Entity Registrant Name	BIOGEN INC.
Entity Central Index Key	0000875045
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Document Type	10-Q
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q3
Amendment Flag	false
Entity Common Stock, Shares Outstanding		201,482,595

Condensed Consolidated Statements of Income (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Total revenues	$ 3,439.0	$ 3,077.8	$ 9,926.6	$ 8,966.9
Cost and expenses:
Cost of sales, excluding amortization of acquired intangible assets	460.8	370.0	1,327.8	1,120.8
Research and development	507.9	446.4	1,985.6	1,666.0
Selling, general and administrative	497.7	433.4	1,515.2	1,361.9
Amortization of acquired intangible assets	281.9	108.9	493.2	674.9
Collaboration profit (loss) sharing	47.5	35.2	129.2	82.5
Acquired in-process research and development	27.5	0.0	112.5	120.0
Restructuring charges	6.0	0.0	9.2	0.0
(Gain) loss on fair value remeasurement of contingent consideration	(87.9)	30.0	(91.6)	61.2
Total cost and expenses	1,741.4	1,423.9	5,481.1	5,087.3
Income from operations	1,697.6	1,653.9	4,445.5	3,879.6
Other income (expense), net	115.1	(44.0)	39.6	(150.6)
Income before income tax expense and equity in loss of investee, net of tax	1,812.7	1,609.9	4,485.1	3,729.0
Income tax expense	369.8	383.8	956.0	892.6
Equity in loss of investee, net of tax	0.0	0.0	0.0	0.0
Net income	1,442.9	1,226.1	3,529.1	2,836.4
Net income (loss) attributable to noncontrolling interests, net of tax	(1.5)	0.0	45.2	(0.1)
Net income attributable to Biogen Inc.	$ 1,444.4	$ 1,226.1	$ 3,483.9	$ 2,836.5
Net income per share:
Basic earnings per share attributable to Biogen Inc.	$ 7.17	$ 5.80	$ 16.86	$ 13.32
Diluted earnings per share attributable to Biogen Inc.	$ 7.15	$ 5.79	$ 16.83	$ 13.30
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Inc.	201.4	211.4	206.6	213.0
Diluted earnings per share attributable to Biogen Inc.	201.9	211.8	207.0	213.3
Product, net
Total revenues	$ 2,780.1	$ 2,622.5	$ 8,061.1	$ 7,642.3
Revenues from anti-CD20 therapeutic programs
Total revenues	511.7	406.5	1,445.3	1,144.2
Other
Total revenues	$ 147.2	$ 48.8	$ 420.2	$ 180.4

Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income attributable to Biogen Inc.	$ 1,444.4	$ 1,226.1	$ 3,483.9	$ 2,836.5
Other comprehensive income:
Unrealized gains (losses) on securities available for sale, net of tax	(0.2)	1.0	(1.8)	6.6
Unrealized gains (losses) on cash flow hedges, net of tax	5.2	(35.5)	109.0	(162.3)
Unrealized gains (losses) on pension benefit obligation, net of tax	(0.2)	0.0	0.2	(0.5)
Currency translation adjustment, net of tax	8.5	43.9	(38.8)	146.7
Total other comprehensive income (loss), net of tax	13.3	9.4	68.6	(9.5)
Comprehensive income attributable to Biogen Inc.	1,457.7	1,235.5	3,552.5	2,827.0
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	(1.5)	0.0	45.2	(0.1)
Comprehensive income	$ 1,456.2	$ 1,235.5	$ 3,597.7	$ 2,826.9

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 2,386.7	$ 1,573.8
Marketable securities	2,041.7	2,115.2
Accounts receivable, net	2,017.3	1,787.0
Due from anti-CD20 therapeutic programs	508.4	532.6
Inventory	916.6	902.7
Other current assets	848.3	962.0
Total current assets	8,719.0	7,873.3
Marketable securities	1,244.5	3,057.3
Property, plant and equipment, net	3,538.9	3,182.4
Intangible assets, net	3,379.0	3,879.6
Goodwill	5,440.1	4,632.5
Deferred tax assets	2,160.9	595.9
Investments and other assets	1,009.8	431.6
Total assets	25,492.2	23,652.6
Current liabilities:
Current portion of notes payable	0.0	3.2
Taxes payable	254.1	68.2
Accounts payable	342.3	395.5
Accrued expenses and other	2,578.5	2,901.3
Total current liabilities	3,174.9	3,368.2
Notes payable	5,931.1	5,935.0
Deferred tax liabilities	1,114.6	122.6
Other long-term liabilities	1,511.8	1,628.7
Total liabilities	11,732.4	11,054.5
Commitments and contingencies
Biogen Idec Inc. shareholders' equity
Preferred stock, par value $0.001 per share	0.0	0.0
Common stock, par value $0.0005 per share	0.1	0.1
Additional paid-in capital	47.8	97.8
Accumulated other comprehensive loss	(248.3)	(318.4)
Retained earnings	16,944.1	15,810.4
Treasury stock, at cost	(2,977.1)	(2,977.1)
Total Biogen Inc. shareholders’ equity	13,766.6	12,612.8
Noncontrolling interests	(6.8)	(14.7)
Total equity	13,759.8	12,598.1
Total liabilities and equity	$ 25,492.2	$ 23,652.6

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Sep. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Common stock, par value	$ 0.0005	$ 0.0005

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash flows from operating activities:
Net income	$ 3,529.1	$ 2,836.4
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation and amortization	686.9	872.0
Acquired in-process research and development	112.5	120.0
Share-based compensation	119.0	97.4
Contingent consideration	(91.6)	61.2
Deferred income taxes	44.8	39.7
Other	(68.2)	65.7
Changes in operating assets and liabilities, net:
Accounts receivable	(254.0)	(225.8)
Inventory	(31.9)	(170.3)
Accrued expenses and other current liabilities	100.7	(504.5)
Income tax assets and liabilities	315.6	170.5
Other changes in operating assets and liabilities, net	(81.0)	(250.2)
Net cash flows provided by operating activities	4,292.3	3,032.7
Cash flows from investing activities:
Proceeds from sales and maturities of marketable securities	7,994.7	4,472.6
Purchases of marketable securities	6,093.8	4,093.9
Contingent consideration paid related to Fumapharm AG acquisition	1,200.0	900.0
Purchases of property, plant and equipment	(544.7)	(636.8)
Acquired in-process research and development	(112.5)	(120.0)
Acquisitions of intangible assets	(3.0)	(910.4)
Purchase of Ionis Pharmaceuticals, Inc. stock	(462.9)	0.0
Other	(1.2)	5.1
Net cash flows used in investing activities	(421.0)	(2,193.6)
Cash flows from financing activities:
Purchases of treasury stock	(3,000.0)	(1,365.4)
Payments related to issuance of stock for share-based compensation arrangements, net	(6.7)	(10.7)
Repayment of borrowings	(3.2)	(3.2)
Net distribution to noncontrolling interest	(36.9)	0.0
Net cash contribution to Bioverativ Inc.	0.0	302.7
Other	11.8	10.1
Net cash flows used in financing activities	(3,035.0)	(1,671.9)
Net increase (decrease) in cash and cash equivalents	836.3	(832.8)
Effect of exchange rate changes on cash and cash equivalents	(23.4)	54.4
Cash and cash equivalents, beginning of the period	1,573.8	2,326.5
Cash and cash equivalents, end of the period	$ 2,386.7	$ 1,548.1

Summary of Significant Accounting Policies

9 Months Ended

Sep. 30, 2018

Accounting Policies [Abstract]

Summary of significant accounting policies

Summary of Significant Accounting Policies

References in these notes to "Biogen," the "company," "we," "us" and "our" refer to Biogen Inc. and its consolidated subsidiaries.

Business Overview

Biogen is a global biopharmaceutical company focused on discovering, developing and delivering worldwide innovative therapies for people living with serious neurological and neurodegenerative diseases, including in our core growth areas of multiple sclerosis (MS) and neuroimmunology, Alzheimer's disease (AD) and dementia, movement disorders and neuromuscular disorders, including spinal muscular atrophy (SMA) and amyotrophic lateral sclerosis (ALS). We are also focused on discovering, developing and delivering worldwide innovative therapies in our emerging growth areas of pain, ophthalmology, neuropsychiatry and acute neurology. In addition, we are employing innovative technologies to discover potential treatments for rare and genetic disorders, including new ways of treating diseases through gene therapy in our core and emerging growth areas. We also manufacture and commercialize biosimilars of advanced biologics.

Our marketed products include TECFIDERA, AVONEX, PLEGRIDY, TYSABRI and FAMPYRA for the treatment of MS, SPINRAZA for the treatment of SMA and FUMADERM for the treatment of severe plaque psoriasis. We also have certain business and financial rights with respect to RITUXAN for the treatment of non-Hodgkin's lymphoma, chronic lymphocytic leukemia (CLL) and other conditions, GAZYVA for the treatment of CLL and follicular lymphoma, OCREVUS for the treatment of primary progressive MS (PPMS) and relapsing MS (RMS) and other potential anti-CD20 therapies pursuant to our collaboration arrangements with Genentech, Inc. (Genentech), a wholly-owned member of the Roche Group. In March 2018 we and AbbVie Inc. (AbbVie) announced the voluntary worldwide withdrawal of ZINBRYTA for RMS. For additional information on our collaboration arrangements with Genentech and AbbVie, please read Note 17, Collaborative and Other Relationships, to these unaudited condensed consolidated financial statements (condensed consolidated financial statements).

We support our drug discovery and development efforts through the commitment of significant resources to discovery, research and development programs and business development opportunities, particularly within our core and emerging growth areas. For nearly two decades we have led in the research and development of new therapies to treat MS, resulting in our leading portfolio of MS treatments. Now our research is focused on additional improvements in the treatment of MS, such as the development of next generation therapies for MS, with a goal to reverse or possibly repair damage caused by the disease. We are also applying our scientific expertise to solve some of the most challenging and complex diseases, including AD, progressive supranuclear palsy, Parkinson's disease, ALS, pain, cognitive impairment associated with schizophrenia (CIAS) and stroke.

Our innovative drug development and commercialization activities are complemented by our biosimilar therapies that expand access to medicines and reduce the cost burden for healthcare systems. We are leveraging our manufacturing capabilities and know-how to develop, manufacture and market biosimilars through Samsung Bioepis Co., Ltd. (Samsung Bioepis), our joint venture with Samsung BioLogics Co. Ltd. (Samsung BioLogics). Under our commercial agreement, we market and sell BENEPALI, an etanercept biosimilar referencing ENBREL, FLIXABI, an infliximab biosimilar referencing REMICADE, and IMRALDI, an adalimumab biosimilar referencing HUMIRA, in the European Union (E.U.). For additional information on our collaboration arrangement with Samsung Bioepis, please read Note 17, Collaborative and Other Relationships, to these condensed consolidated financial statements.

Basis of Presentation

In the opinion of management, our condensed consolidated financial statements include all adjustments, consisting of normal recurring accruals, necessary for a fair presentation of our financial statements for interim periods in accordance with accounting principles generally accepted in the United States (U.S. GAAP). The information included in this quarterly report on Form 10-Q should be read in conjunction with our audited consolidated financial statements and the accompanying notes included in our Annual Report on Form 10-K for the year ended December 31, 2017 (2017 Form 10-K). Our accounting policies are described in the “Notes to Consolidated Financial Statements” in our 2017 Form 10-K and updated, as necessary, in this report. The year-end condensed consolidated balance sheet data presented for comparative purposes was derived from our audited financial statements but does not include all disclosures required by U.S. GAAP. The results of operations for the three and nine months ended September 30, 2018, are not necessarily indicative of the operating results for the full year or for any other subsequent interim period.

We operate as one operating segment, focused on discovering, developing and delivering worldwide innovative therapies for people living with serious neurological and neurodegenerative diseases.

Consolidation

Our condensed consolidated financial statements reflect our financial statements, those of our wholly-owned subsidiaries and those of certain variable interest entities where we are the primary beneficiary. For consolidated entities where we own or are exposed to less than 100% of the economics, we record net income (loss) attributable to noncontrolling interests in our condensed consolidated statements of income equal to the percentage of the economic or ownership interest retained in such entities by the respective noncontrolling parties. Intercompany balances and transactions are eliminated in consolidation.

In determining whether we are the primary beneficiary of an entity, we apply a qualitative approach that determines whether we have both (1) the power to direct the economically significant activities of the entity and (2) the obligation to absorb losses of, or the right to receive benefits from, the entity that could potentially be significant to that entity. These considerations impact the way we account for our existing collaborative relationships and other arrangements. We continuously assess whether we are the primary beneficiary of a variable interest entity as changes to existing relationships or future transactions may result in us consolidating or deconsolidating one or more of our collaborators or partners.

Use of Estimates

The preparation of our condensed consolidated financial statements requires us to make estimates, judgments and assumptions that may affect the reported amounts of assets, liabilities, equity, revenues and expenses and related disclosure of contingent assets and liabilities. On an on-going basis we evaluate our estimates, judgments and methodologies. We base our estimates on historical experience and on various other assumptions that we believe are reasonable, the results of which form the basis for making judgments about the carrying values of assets, liabilities and equity and the amount of revenues and expenses. Actual results may differ from these estimates.

New Accounting Pronouncements

From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board (FASB) or other standard setting bodies that we adopt as of the specified effective date. Unless otherwise discussed below, we do not believe that the adoption of recently issued standards have or may have a material impact on our condensed consolidated financial statements and disclosures.

Revenue Recognition

In May 2014 the FASB issued Accounting Standards Update (ASU) No. 2014-09, Revenue from Contracts with Customers (Topic 606), which supersedes all existing revenue recognition requirements, including most industry specific guidance. This new standard requires a company to recognize revenues when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services.

The FASB subsequently issued the following amendments to ASU 2014-09 that have the same effective date and transition date: ASU No. 2016-08, Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations; ASU No. 2016-10, Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing; ASU No. 2016-12, Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients; and ASU No. 2016-20, Technical Corrections and Improvements to Topic 606, Revenue from Contracts with Customers. We adopted these amendments with ASU 2014-09 (collectively, the new revenue standards).

The new revenue standards became effective for us on January 1, 2018, and were adopted using the modified retrospective method. The adoption of the new revenue standards as of January 1, 2018, did not change our revenue recognition as the majority of our revenues continue to be recognized when the customer takes control of our product. As we did not identify any accounting changes that impacted the amount of reported revenues with respect to our product revenues, revenues from anti-CD20 therapeutic programs or other revenues, no adjustment to retained earnings was required upon adoption. However, the adoption of the new revenue standards may result in a change in the timing of revenue recognition related to certain of our contract manufacturing activities based upon the terms of the underlying agreements.

Under the new revenue standards, we recognize revenues when our customer obtains control of promised goods or services, in an amount that reflects the consideration which we expect to receive in exchange for those goods or services. We recognize revenues following the five-step model prescribed under ASU 2014-09: (i) identify contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenues when (or as) we satisfy the performance obligation.

Product Revenues

In the United States (U.S.) we sell our products primarily to wholesale distributors and specialty pharmacy providers. In other countries, we sell our products primarily to wholesale distributors, hospitals, pharmacies and other third-party distribution partners. These customers subsequently resell our products to health care providers and patients. In addition, we enter into arrangements with health care providers and payors that provide for government-mandated or privately-negotiated discounts and allowances related to our products.

Revenues from product sales are recognized when the customer obtains control of our product, which occurs at a point in time, typically upon delivery to the customer. We expense incremental costs of obtaining a contract as and when incurred if the expected amortization period of the asset that we would have recognized is one year or less or the amount is immaterial.

Reserves for Discounts and Allowances

Revenues from product sales are recorded net of reserves established for applicable discounts and allowances that are offered within contracts with our customers, health care providers or payors, including those associated with the implementation of pricing actions in certain of the international markets in which we operate. Our process for estimating reserves established for these variable consideration components do not differ materially from our historical practices.

Product revenue reserves, which are classified as a reduction in product revenues, are generally characterized in the following categories: discounts, contractual adjustments and returns.

These reserves are based on estimates of the amounts earned or to be claimed on the related sales and are classified as reductions of accounts receivable (if the amount is payable to our customer) or a liability (if the amount is payable to a party other than our customer). Our estimates of reserves established for variable consideration are calculated based upon a consistent application of our methodology utilizing the expected value method. These estimates reflect our historical experience, current contractual and statutory requirements, specific known market events and trends, industry data and forecasted customer buying and payment patterns. The transaction price, which includes variable consideration reflecting the impact of discounts and allowances, may be subject to constraint and is included in the net sales price only to the extent that it is probable that a significant reversal of the amount of the cumulative revenues recognized will not occur in a future period. Actual amounts may ultimately differ from our estimates. If actual results vary, we adjust these estimates, which could have an effect on earnings in the period of adjustment.

In addition to discounts, rebates and product returns, we also maintain certain customer service contracts with distributors and other customers in the distribution channel that provide us with inventory management, data and distribution services, which are generally reflected as a reduction of revenues. To the extent we can demonstrate a separable benefit and fair value for these services we classify these payments in selling, general and administrative expenses.

For additional information on our revenues, please read Note 4, Revenues, to these condensed consolidated financial statements.

Revenues from Anti-CD20 Therapeutic Programs

Our collaboration with Genentech is within the scope of Accounting Standards Codification 808, Collaborative Agreements, which provides guidance on the presentation and disclosure of collaborative arrangements. Our share of the pre-tax co-promotion profits on RITUXAN and GAZYVA and royalty revenues on the sale of OCREVUS resulted from an exchange of a license. As we do not have any future performance obligations under the license or collaboration agreement, revenues are recognized as the underlying sales occur.

Revenues from anti-CD20 therapeutic programs consist of:


(i)	our share of pre-tax profits and losses in the U.S. for RITUXAN and GAZYVA; and


(ii)	other revenues from anti-CD20 therapeutic programs, which primarily consist of our share of pre-tax co-promotion profits on RITUXAN in Canada and royalty revenues on sales of OCREVUS.

For additional information on our relationship with Genentech, please read Note 20, Collaborative and Other Relationships, to our consolidated financial statements included in our 2017 Form 10-K.

Collaborative and Other Relationships

We have a number of significant collaborative and other third-party relationships for revenues and for the development, regulatory approval, commercialization and marketing of certain of our products and product candidates. Where we are the principal on sales transactions with third parties, we recognize revenues, cost of sales and operating expenses on a gross basis in their respective lines in our condensed consolidated statements of income. Where we are not the principal on sales transactions with third parties, we record our share of the revenues, cost of sales and operating expenses on a net basis in collaborative and other relationships included in other revenues in our condensed consolidated statements of income.

Our development and commercialization arrangements with AbbVie, Genentech and Samsung Bioepis represent collaborative arrangements as each party is an active participant in one or more joint operating activities and is exposed to significant risks and rewards of these arrangements. These arrangements resulted from an exchange of a license and utilize the sales and usage-based royalty exception. Therefore, revenues relating to royalties or profit-sharing amounts received are recognized as the underlying sales occur.

For additional information on our collaboration arrangements with AbbVie, Genentech and Samsung Bioepis, please read Note 17, Collaborative and Other Relationships, to these condensed consolidated financial statements.

Royalty Revenues

We receive royalty revenues on sales by our licensees of other products covered under patents that we own. We do not have future performance obligations under these license arrangements. We record these revenues based on estimates of the sales that occurred during the relevant period as a component of other revenues. The relevant period estimates of sales are based on interim data provided by licensees and analysis of historical royalties that have been paid to us, adjusted for any changes in facts and circumstances, as appropriate. Differences between actual and estimated royalty revenues are adjusted for in the period in which they become known, typically the following quarter. Historically, adjustments have not been material when compared to actual amounts paid by licensees.

Other Corporate Revenues

We record other corporate revenues primarily from amounts earned under contract manufacturing agreements. Revenues under contract manufacturing agreements are recognized when the customer obtains control of the product, which may occur at a point in time or over time depending on the terms and conditions of the agreement.

Accounts Receivable

The majority of our accounts receivable arise from product sales and primarily represent amounts due from our wholesale and other third-party distributors, public hospitals, pharmacies and other government entities and have standard payment terms that generally require payment within 30 to 90 days.

We do not adjust our receivables for the effects of a significant financing component at contract inception if we expect to collect the receivables in one year or less from the time of sale.

In countries where we have experienced a pattern of payments extending beyond our contractual payment term and we expect to collect receivables greater than one year from the time of sale, we have assessed whether the customer has a significant financing component and discounted our receivables and reduced related revenues over the period of time that we estimate those amounts will be paid using the country’s market-based borrowing rate for such period. The related receivables are classified at the time of sale as non-current assets. We accrete interest income on these receivables, which is recognized as a component of other income (expense), net in our condensed consolidated statements of income.

We provide reserves against trade receivables for estimated losses that may result from a customer’s inability to pay. Amounts determined to be uncollectible are charged or written-off against the reserve.

The adoption of the new revenue standards did not change our historical accounting methods for our accounts receivable.

Financial Instruments

In January 2016 the FASB issued ASU No. 2016-01, Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities. This new standard amends certain aspects of accounting and disclosure requirements for financial instruments, including the requirement that equity investments with readily determinable fair values are to be measured at fair value with any changes in fair value recognized in a company's results of operations. This new standard does not apply to investments accounted for under the equity method of accounting or those investments that result in consolidation of the investee. Equity investments that do not have readily determinable fair values may be measured at fair value or at cost minus impairment adjusted for changes in observable prices. A financial liability that is measured at fair value in accordance with the fair value option is required to be presented separately in other comprehensive income for the portion of the total change in the fair value resulting from change in the instrument-specific credit risk. In addition, a valuation allowance should be evaluated on deferred tax assets related to available-for-sale debt securities in combination with other deferred tax assets.

We adopted this new standard on January 1, 2018, using the modified retrospective method, and recognized a $1.3 million net adjustment to retained earnings reflecting the cumulative impact for the accounting changes made upon adoption. The adoption of this new standard resulted in a change in the income statement classification with respect to where we recognize changes in fair value related to certain equity security investments. Prior to the adoption of ASU 2016-01, we recognized changes in fair value in accumulated other comprehensive income (loss), net. Upon the adoption of ASU 2016-01, we recognize changes in fair value in other income (expense), net.

Leasing

In February 2016 the FASB issued ASU No. 2016-02, Leases (Topic 842). This new standard establishes a right-of-use model that requires all lessees to recognize right-of-use assets and liabilities on their balance sheet that arise from leases with terms longer than 12 months as well as provide disclosures with respect to certain qualitative and quantitative information related to their leasing arrangements. This new standard will become effective for us on January 1, 2019.

The FASB subsequently issued the following amendments to ASU 2016-02, which have the same effective date and transition date of January 1, 2019, and which we collectively refer to as the new leasing standards:


•	ASU No. 2018-10, Codification Improvements to Topic 842, Leases, which amends certain narrow aspects of the guidance issued in ASU 2016-02.


•	ASU No. 2018-11, Leases (Topic 842): Targeted Improvements, which allows for a transition approach to initially apply ASU 2016-02 at the adoption date and recognize a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption as well as an additional practical expedient for lessors to not separate non-lease components from the associated lease component.

We are in the process of reviewing our existing lease contracts and continue to evaluate the impact that the new leasing standards may have on our consolidated results of operations, financial position and disclosures. We expect that the adoption of the new leasing standards will result in the recognition of material right-of-use assets and liabilities in our condensed consolidated balance sheets. The adoption of the new leasing standards is not expected to have a material impact to our condensed consolidated statements of income.

We will adopt the new leasing standards using a modified retrospective transition approach to be applied to leases existing as of, or entered into after, January 1, 2019.

Income Taxes

In October 2016 the FASB issued ASU No. 2016-16, Income Taxes (Topic 740): Intra-Entity Transfer of Assets Other Than Inventory. This new standard eliminates the deferral of the tax effects of intra-entity asset transfers other than inventory. As a result, the income tax consequences from the intra-entity transfer of an asset other than inventory and associated changes to deferred taxes will be recognized when the transfer occurs.

We adopted this new standard on January 1, 2018, using the modified retrospective method, through a cumulative-effect adjustment to retained earnings as of that date. Upon adoption, we recognized additional net deferred tax assets of approximately $0.5 billion offset by a corresponding net increase to retained earnings of approximately $0.5 billion. We will recognize incremental deferred income tax expense thereafter as these deferred tax assets and liabilities are utilized.

For additional information on our income taxes, please read Note 15, Income Taxes, to these condensed consolidated financial statements.

Net Periodic Pension Cost

In March 2017 the FASB issued ASU No. 2017-07, Compensation - Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost. This new standard requires that an employer disaggregate the service cost component from the other components of net benefit cost. This new standard also provides explicit guidance on how to present the service cost component and the other components of net benefit cost in the statements of income and allows only the service cost component of net benefit cost to be eligible for capitalization. The other components of the net periodic benefit cost must be presented separately from the line items that include service cost and outside of any subtotal of operating income on our condensed consolidated statements of income. We adopted this new standard on January 1, 2018, using the retrospective method.

As a result of the adoption of this new standard, the other components of the net periodic benefit cost, which we previously presented as a component of operating income, are now classified in other income (expense), net in our condensed consolidated statements of income. For the three and nine months ended September 30, 2017, $0.4 million and $1.2 million, respectively, were reclassified from operating income to other income (expense), net in our condensed consolidated statements of income to conform to our current year presentation.

Debt Securities

In March 2017 the FASB issued ASU No. 2017-08, Receivables - Nonrefundable Fees and Other Costs (Subtopic 310-20): Premium Amortization on Purchased Callable Debt Securities. This new standard amends the amortization period for certain purchased callable debt securities held at a premium by shortening the amortization period to the earliest call date. This new standard will be effective for us on January 1, 2019. Upon adoption, we will decrease our marketable securities for the amended amortization period with a corresponding adjustment to retained earnings. We do not expect that the adoption of this new standard will have a material impact on our financial position or results of operations due to the shortening of the amortization period. The ultimate impact of adopting this new standard will depend on our marketable debt securities as of the adoption date.

Derivative Instruments and Hedging Activities

In August 2017 the FASB issued ASU No. 2017-12, Derivatives and Hedging (Topic 815): Targeted Improvements to Accounting for Hedging Activities. This new standard provides guidance to better align an entity’s risk management activities and financial reporting for hedging relationships through changes to both the designation and measurement guidance for qualifying hedging relationships and the presentation of hedge results. This new standard expands and refines hedge accounting for both non-financial and financial risk components and aligns the recognition and presentation of the effects of the hedging instrument and the hedged item in the financial statements.

We adopted this new standard on January 1, 2018, using the modified retrospective method, which did not have an impact on our financial position or results of operations; however, the adoption of this new standard resulted in additional disclosures and a change in the income statement classification with respect to where we recognize ineffective hedge transaction gains and losses. Prior to the adoption of ASU 2017-12 on January 1, 2018, to the extent ineffective, hedge transaction gains and losses were reported in other income (expense), net. Effective January 1, 2018, we recognize all fair value changes of derivatives in earnings, including any ineffective portion, in the same line item in our condensed consolidated statements of income that has been impacted by the hedged item.

We recognize all derivative instruments as either assets or liabilities at fair value in our condensed consolidated balance sheets. Changes in the fair value of our derivative instruments are recognized each period in current earnings or accumulated other comprehensive income (loss), depending on whether the derivative instrument is designated as part of a hedge transaction and, if so, the type of hedge transaction. We classify the cash flows from these instruments in the same category as the cash flows from the hedged items. We do not hold or issue derivative instruments for trading or speculative purposes.

We assess at inception and on an on-going basis, whether the derivative instruments that are used in hedging transactions are highly effective in offsetting the changes in cash flows or fair values of the hedged items. We exclude the forward points portion of the derivative instrument used in a hedging transaction from the effectiveness test and record the fair value gain or loss related to this portion each period in the same line item as the underlying hedged item. If we determine that a forecasted transaction is no longer probable of occurring, we discontinue hedge accounting for the affected portion of the hedge instrument and any related unrealized gain or loss on the contract is recognized in current earnings.

For additional information on our derivative instruments and hedging activities, please read Note 9, Derivative Instruments, to these condensed consolidated financial statements.

Fair Value Measurements

In August 2018 the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement. This new standard modifies certain disclosure requirements on fair value measurements. This new standard will be effective for us on January 1, 2020. We do not expect that the adoption of this new standard will have a material impact on our disclosures.

Acquisitions	9 Months Ended
Acquisitions	Sep. 30, 2018
Business Combinations [Abstract]
Acquisitions	Acquisitions BIIB100 Acquisition In January 2018 we acquired the Phase 1 ready investigational oral compound BIIB100 (formerly known as KPT-350) for the treatment of certain neurological and neurodegenerative conditions, primarily in ALS, from Karyopharm Therapeutics Inc. (Karyopharm). BIIB100 is a novel therapeutic candidate that works by inhibiting a protein known as XPO1, with the goal of reducing inflammation and neurotoxicity, along with increasing neuroprotective responses. We accounted for this transaction as an asset acquisition as the value being acquired relates to a single asset. In connection with the closing of this transaction, we made an upfront payment of $10.0 million to Karyopharm, which was recorded as acquired in-process research and development in our condensed consolidated statements of income as BIIB100 has not yet reached technological feasibility. We may also pay Karyopharm up to $207.0 million in additional milestone payments as well as tiered royalties on potential net commercial sales in the mid-single digit to low-teen percentages. BIIB104 Acquisition In April 2018 we acquired BIIB104 (formerly known as PF-04958242) from Pfizer Inc. (Pfizer). BIIB104 is a first-in-class, Phase 2b ready AMPA receptor potentiator for CIAS, representing our first program in neuropsychiatry. AMPA receptors mediate fast excitatory synaptic transmission in the central nervous system, a process which can be disrupted in a number of neurological and psychiatric diseases, including schizophrenia. We accounted for this transaction as an asset acquisition as the value being acquired primarily relates to a single asset. In connection with the closing of this transaction, we made an upfront payment of $75.0 million to Pfizer, which was recorded as acquired in-process research and development in our condensed consolidated statements of income as BIIB104 has not yet reached technological feasibility. We may also pay Pfizer up to $515.0 million in additional development and commercialization milestone payments as well as tiered royalties on potential net commercial sales in the low to mid-teen percentages. The next expected milestone would be $10.0 million upon the dosing of the first patient in the Phase 2b study, which will be recorded as research and development expense in our condensed consolidated statements of income. TMS Co., Ltd. In June 2018 we entered into an exclusive option agreement with TMS Co., Ltd. (TMS) granting us the option to acquire TMS-007, a plasminogen activator with a novel mechanism of action (MOA) associated with breaking down blood clots, which is in Phase 2 development in Japan, and backup compounds for the treatment of stroke. In exchange for the purchase option, we made a $4.0 million upfront payment to TMS, which was recorded as research and development expense in our condensed consolidated statements of income as TMS-007 has not yet reached technological feasibility. If we exercise the purchase option, we will make an additional payment of $18.0 million upon closing of the asset acquisition, which will be recorded as acquired in-process research and development expense in our condensed consolidated statements of income as TMS-007 will not have reached technological feasibility at that time. In addition, we may also pay TMS up to $335.0 million in additional development and commercialization milestone payments as well as tiered royalties on potential net commercial sales in the high-single digit to low-teen percentages. If we exercise the purchase option, consummation of the asset acquisition may be subject to the expiration of the applicable waiting period under the Hart-Scott-Rodino Antitrust Improvements Act of 1976 in the U.S. BIIB110 Acquisition In July 2018 we acquired BIIB110 (formerly known as ALG-801) (Phase 1a) and ALG-802 (preclinical) from AliveGen Inc. (AliveGen). BIIB110 and ALG-802 represent novel ways of targeting the myostatin pathway. We initially plan to study BIIB110 in multiple neuromuscular indications, including SMA and ALS. We accounted for this transaction as an asset acquisition as the value being acquired primarily relates to a single asset. In connection with the closing of this transaction, we made an upfront payment of $27.5 million to AliveGen, which was recorded as acquired in-process research and development in our condensed consolidated statements of income as BIIB110 has not yet reached technological feasibility. We may also pay AliveGen up to $535.0 million in additional development and commercialization milestones.

Restructuring Restructuring	9 Months Ended
Restructuring Restructuring	Sep. 30, 2018
Restructuring and Related Activities [Abstract]
Restructuring	Restructuring 2017 Corporate Strategy In October 2017, in connection with creating a leaner and simpler operating model, we approved a corporate restructuring program intended to streamline our operations and reallocate resources. For the three and nine months ended September 30, 2018, we recognized restructuring charges of $6.0 million and $9.2 million, respectively, in our condensed consolidated statements of income. These restructuring charges were primarily related to severance. We previously recognized restructuring charges of $0.9 million in our condensed consolidated statements of income during the fourth quarter of 2017, which were primarily related to severance. Restructuring charges incurred to date under this program are expected to be substantially paid in cash by the end of 2018.

Revenues Revenues

9 Months Ended

Sep. 30, 2018

Revenue from Contract with Customer [Abstract]

Revenues

Product Revenues

Revenues by product are summarized as follows:



	For the Three Months Ended September 30,
(In millions)	2018						2017
	United States		Rest of World		Total		United States		Rest of World		Total
Multiple Sclerosis:
TECFIDERA	$	842.1	$	247.9	$	1,090.0	$	836.3	$	233.3	$	1,069.6
Interferon*	421.5		168.6		590.1		473.3		188.7		662.0
TYSABRI	253.0		217.2		470.2		266.8		202.6		469.4
FAMPYRA	—		22.5		22.5		—		24.3		24.3
ZINBRYTA	—		—		—		—		14.2		14.2
Spinal Muscular Atrophy:
SPINRAZA	223.9		243.8		467.7		197.6		73.3		270.9
Other Product Revenues:
FUMADERM	—		4.8		4.8		—		10.7		10.7
BENEPALI	—		123.4		123.4		—		99.2		99.2
FLIXABI	—		11.4		11.4		—		2.2		2.2
Total product revenues	$	1,740.5	$	1,039.6	$	2,780.1	$	1,774.0	$	848.5	$	2,622.5

*Interferon includes AVONEX and PLEGRIDY.



	For the Nine Months Ended September 30,
(In millions)	2018						2017
	United States		Rest of World		Total		United States		Rest of World		Total
Multiple Sclerosis:
TECFIDERA	$	2,396.8	$	766.9	$	3,163.7	$	2,462.4	$	676.0	$	3,138.4
Interferon*	1,237.5		528.4		1,765.9		1,439.8		561.1		2,000.9
TYSABRI	768.2		631.3		1,399.5		861.7		648.7		1,510.4
FAMPYRA	—		69.9		69.9		—		67.4		67.4
ZINBRYTA	—		1.4		1.4		—		41.0		41.0
Spinal Muscular Atrophy:
SPINRAZA	617.8		636.5		1,254.3		438.8		82.4		521.2
Hemophilia:
ELOCTATE	—		—		—		42.2		6.2		48.4
ALPROLIX	—		—		—		21.0		5.0		26.0
Other Product Revenues:
FUMADERM	—		17.3		17.3		—		30.7		30.7
BENEPALI	—		359.9		359.9		—		253.2		253.2
FLIXABI	—		29.2		29.2		—		4.7		4.7
Total product revenues	$	5,020.3	$	3,040.8	$	8,061.1	$	5,265.9	$	2,376.4	$	7,642.3

*Interferon includes AVONEX and PLEGRIDY.

We recognized revenues from two wholesalers accounting for 30.7% and 19.3% of gross product revenues for the three months ended September 30, 2018, and 32.4% and 18.0% of gross product revenues for the nine months ended September 30, 2018.

We recognized revenues from two wholesalers accounting for 33.7% and 20.8% of gross product revenues for the three months ended September 30, 2017, and 34.9% and 21.0% of gross product revenues for the nine months ended September 30, 2017.

An analysis of the change in reserves for discounts and allowances is summarized as follows:



(In millions)	Discounts			Contractual Adjustments			Returns			Total
Balance, as of December 31, 2017	$	109.6		$	606.0		$	46.0		$	761.6
Current provisions relating to sales in current year	498.9			1,949.6			18.0			2,466.5
Adjustments relating to prior years	(0.3		)	(7.0		)	0.1			(7.2		)
Payments/credits relating to sales in current year	(373.7		)	(1,285.8		)	(0.8		)	(1,660.3		)
Payments/credits relating to sales in prior years	(107.8		)	(495.2		)	(21.8		)	(624.8		)
Balance, as of September 30, 2018	$	126.7		$	767.6		$	41.5		$	935.8

The total reserves above, which are included in our condensed consolidated balance sheets, are summarized as follows:



(In millions)	As of September 30, 2018		As of December 31, 2017
Reduction of accounts receivable	$	177.3	$	189.6
Component of accrued expenses and other	758.5		572.0
Total reserves	$	935.8	$	761.6

Revenues from Anti-CD20 Therapeutic Programs

Revenues from anti-CD20 therapeutic programs are summarized as follows:



	For the Three Months Ended September 30,				For the Nine Months Ended September 30,
(In millions)	2018		2017		2018		2017
Biogen’s share of pre-tax profits in the U.S. for RITUXAN and GAZYVA	$	358.0	$	325.1	$	1,066.6	$	996.1
Other revenues from anti-CD20 therapeutic programs	153.7		81.4		378.7		148.1
Total revenues from anti-CD20 therapeutic programs	$	511.7	$	406.5	$	1,445.3	$	1,144.2

For additional information on our relationship with Genentech, please read Note 20, Collaborative and Other Relationships, to our consolidated financial statements included in our 2017 Form 10-K.

Other Revenues

Other revenues are summarized as follows:



	For the Three Months Ended September 30,						For the Nine Months Ended September 30,
(In millions)	2018			2017			2018			2017
Revenues from collaborative and other relationships:
AbbVie	$	(0.7	)	$	(2.8	)	$	(7.9	)	$	(12.6	)
Samsung Bioepis and other	48.1			9.0			80.7			34.1
Other royalty and corporate revenues:
Royalty	7.9			11.8			35.8			49.1
Other corporate	91.9			30.8			311.6			109.8
Total other revenues	$	147.2		$	48.8		$	420.2		$	180.4

For additional information on our collaboration arrangements with AbbVie and Samsung Bioepis, please read Note 17, Collaborative and Other Relationships, to these condensed consolidated financial statements.

Inventory

9 Months Ended

Sep. 30, 2018

Inventory Disclosure [Abstract]

Inventory

The components of inventory are summarized as follows:



(In millions)	As of September 30, 2018		As of December 31, 2017
Raw materials	$	181.8	$	162.4
Work in process	608.0		605.7
Finished goods	136.5		157.4
Total inventory	$	926.3	$	925.5

Balance Sheet Classification:
Inventory	$	916.6	$	902.7
Investments and other assets	9.7		22.8
Total inventory	$	926.3	$	925.5

Long-term inventory, which primarily consists of work in process, is included in investments and other assets in our condensed consolidated balance sheets.

Intangible Assets and Goodwill

9 Months Ended

Sep. 30, 2018

Goodwill and Intangible Assets Disclosure [Abstract]

Intangible Assets and Goodwill

Intangible Assets

Intangible assets, net of accumulated amortization, impairment charges and adjustments, are summarized as follows:



		As of September 30, 2018							As of December 31, 2017
(In millions)	Estimated Life	Cost		Accumulated Amortization			Net		Cost		Accumulated Amortization			Net
Out-licensed patents	13-23 years	$	543.3	$	(541.6	)	$	1.7	$	543.3	$	(535.6	)	$	7.7
Developed technology	15-23 years	3,005.3		(2,724.0		)	281.3		3,005.3		(2,689.0		)	316.3
In-process research and development	Indefinite until commercialization	480.9		—			480.9		680.6		—			680.6
Trademarks and tradenames	Indefinite	64.0		—			64.0		64.0		—			64.0
Acquired and in-licensed rights and patents	4-18 years	3,974.2		(1,423.1		)	2,551.1		3,971.4		(1,160.4		)	2,811.0
Total intangible assets		$	8,067.7	$	(4,688.7	)	$	3,379.0	$	8,264.6	$	(4,385.0	)	$	3,879.6

For the three and nine months ended September 30, 2018, amortization of acquired intangible assets totaled $281.9 million and $493.2 million, respectively, compared to $108.9 million and $674.9 million, respectively, in the prior year comparative periods.

Amortization of acquired intangible assets for the three and nine months ended September 30, 2018, compared to the same periods in 2017, reflects the impact of impairment charges related to certain in-process research and development (IPR&D) assets associated with our vixotrigine (BIIB074) program totaling $189.3 million, as discussed below.

Amortization of acquired intangible assets for the nine months ended September 30, 2017, reflects the impact of a $328.2 million impairment charge recognized in the first quarter of 2017 related to our U.S. and rest of world licenses to Forward Pharma A/S' (Forward Pharma) intellectual property, including Forward Pharma's intellectual property related to TECFIDERA, as discussed below.

Developed Technology

Developed technology primarily relates to our AVONEX product, which was recorded in connection with the merger of Biogen, Inc. and IDEC Pharmaceuticals Corporation in 2003. The net book value of this asset as of September 30, 2018, was $275.3 million.

IPR&D - Vixotrigine

IPR&D represents the fair value assigned to research and development assets that we acquired and had not yet reached technological feasibility at the date of acquisition. We review amounts capitalized as acquired IPR&D for impairment annually and whenever events or changes in circumstances indicate to us that the carrying value of the assets might not be recoverable.

During the third quarter of 2018 we completed a Phase 2b study for vixotrigine in painful lumbosacral radiculopathy (PLSR). The study did not meet its primary or secondary efficacy endpoints and we will discontinue development in PLSR. As a result, we recognized an impairment charge of approximately $60.0 million during the third quarter of 2018 to reduce the fair value of the IPR&D intangible asset to zero.

In addition, we have delayed the initiation of the Phase 3 studies of vixotrigine in trigeminal neuralgia (TGN) as we await the outcome of ongoing interactions with the U.S. Food and Drug Administration (FDA) regarding the design of the Phase 3 studies, a more detailed review of the data from the Phase 2b study of vixotrigine in PLSR and insights from the Phase 2 study of vixotrigine in small fiber neuropathy. We have reassessed the fair value of the TGN program using reduced expected lifetime revenues, higher expected clinical development costs and a lower cumulative probability of success. As a result, we recognized an impairment charge of $129.3 million during the third quarter of 2018 to reduce the fair value of the TGN IPR&D intangible asset to $41.8 million. We also adjusted the value of our contingent consideration obligations related to this program to reflect the lower cumulative probabilities of success resulting in a gain of $89.6 million in the third quarter of 2018.

The IPR&D impairment charges were included in amortization of acquired intangible assets and the gain resulting from the remeasurement of our contingent consideration obligation was recorded in (gain) loss on fair value remeasurement of contingent consideration in our condensed consolidated statements of income. The fair values of the intangible assets and contingent consideration obligations were based on a probability-adjusted discounted cash flow calculation using Level 3 fair value measurements and inputs including estimated revenues, costs and probabilities of success.

We may recognize additional impairment charges in the future depending upon our ability to advance vixotrigine for the treatment of TGN or other indications.

Acquired and In-licensed Rights and Patents

Acquired and in-licensed rights and patents primarily relate to our acquisition of all remaining rights to TYSABRI from Elan Corporation plc and our U.S. and rest of world licenses to Forward Pharma's intellectual property, including Forward Pharma's intellectual property related to TECFIDERA. The net book values of the TYSABRI and TECFIDERA assets as of September 30, 2018, were $2,073.6 million and $262.6 million, respectively.

TECFIDERA License Rights

In January 2017 we entered into a settlement and license agreement among Biogen Swiss Manufacturing GmbH, Biogen International Holding Ltd., Forward Pharma and certain related parties, which was effective as of February 1, 2017. Pursuant to this agreement, we obtained U.S. and rest of world licenses to Forward Pharma's intellectual property, including Forward Pharma's intellectual property related to TECFIDERA. In exchange, we paid Forward Pharma $1.25 billion in cash, of which $795.2 million was recognized as an intangible asset in the first quarter of 2017.

We have two intellectual property disputes with Forward Pharma, one in the U.S. and one in the E.U., concerning intellectual property related to TECFIDERA.

In March 2017 the U.S. intellectual property dispute was decided in our favor. Forward Pharma appealed to the U.S. Court of Appeals for the Federal Circuit and the appeal is pending. We evaluated the recoverability of the U.S. asset acquired from Forward Pharma and recorded a $328.2 million impairment charge in the first quarter of 2017 to adjust the carrying value of the acquired U.S. asset to fair value reflecting the impact of the developments in the U.S. legal dispute.

In March 2018 the European Patent Office (EPO) revoked Forward Pharma's European Patent No. 2 801 355. Forward Pharma has filed an appeal to the Technical Board of Appeal of the EPO and the appeal is pending.

Based upon our assessment of these rulings, we continue to amortize the remaining net book value of the U.S. and rest of world intangible assets in our condensed consolidated statements of income utilizing an economic consumption model.

For additional information on these disputes, please read Note 21, Litigation, to our consolidated financial statements included in our 2017 Form 10-K.

Estimated Future Amortization of Intangible Assets

Our amortization expense is based on the economic consumption and impairment of intangible assets. Our most significant intangible assets are related to our TECFIDERA, AVONEX and TYSABRI products and programs acquired through business combinations. Annually, during our long-range planning cycle, we perform an analysis of anticipated lifetime revenues of our TECFIDERA, AVONEX and TYSABRI products. This analysis is also updated whenever events or changes in circumstances would significantly affect the anticipated lifetime revenues of any of these products. Impairments are recorded in the period in which they are incurred.

Our most recent long-range planning cycle was completed in the third quarter of 2018. Based upon this analysis, the estimated future amortization of acquired intangible assets for the next five years is expected to be as follows:



(In millions)	As of September 30, 2018
2018 (remaining three months)	$	91.7
2019	367.0
2020	366.8
2021	249.4
2022	250.5
2023	230.0

Goodwill

The following table provides a roll forward of the changes in our goodwill balance:



(In millions)	As of September 30, 2018
Goodwill, beginning of period	$	4,632.5
Increase to goodwill	810.7
Other	(3.1		)
Goodwill, end of period	$	5,440.1

The increase to goodwill during the nine months ended September 30, 2018, was related to $900.0 million in contingent milestones achieved (exclusive of $89.3 million in tax benefits) and payable to the former shareholders of Fumapharm AG and holders of their rights.

For additional information on future contingent payments to the former shareholders of Fumapharm AG and holders of their rights, please read Note 22, Commitments and Contingencies, to our consolidated financial statements included in our 2017 Form 10-K.

Other includes changes in foreign currency exchange rate fluctuations. As of September 30, 2018, we had no accumulated impairment losses related to goodwill.

Fair Value Measurements

9 Months Ended

Sep. 30, 2018

Fair Value Disclosures [Abstract]

Fair Value Measurements

The tables below present information about our assets and liabilities that are regularly measured and carried at fair value and indicate the level within the fair value hierarchy of the valuation techniques we utilized to determine such fair value:



As of September 30, 2018 (In millions)	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	$	1,900.9	$	—	$	1,900.9	$	—
Marketable debt securities:
Corporate debt securities	1,951.6		—		1,951.6		—
Government securities	1,060.9		—		1,060.9		—
Mortgage and other asset backed securities	273.7		—		273.7		—
Marketable equity securities	624.8		95.6		529.2		—
Derivative contracts	35.1		—		35.1		—
Plan assets for deferred compensation	26.4		—		26.4		—
Total	$	5,873.4	$	95.6	$	5,777.8	$	—
Liabilities:
Derivative contracts	$	31.2	$	—	$	31.2	$	—
Contingent consideration obligations	412.0		—		—		412.0
Total	$	443.2	$	—	$	31.2	$	412.0



As of December 31, 2017 (In millions)	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	$	1,229.4	$	—	$	1,229.4	$	—
Marketable debt securities:
Corporate debt securities	2,609.8		—		2,609.8		—
Government securities	1,919.3		—		1,919.3		—
Mortgage and other asset backed securities	643.4		—		643.4		—
Marketable equity securities	11.8		11.8		—		—
Derivative contracts	2.7		—		2.7		—
Plan assets for deferred compensation	28.5		—		28.5		—
Total	$	6,444.9	$	11.8	$	6,433.1	$	—
Liabilities:
Derivative contracts	$	111.3	$	—	$	111.3	$	—
Contingent consideration obligations	523.6		—		—		523.6
Total	$	634.9	$	—	$	111.3	$	523.6

There have been no impairments of our assets measured and carried at fair value during the three and nine months ended September 30, 2018. In addition, there were no changes in valuation techniques or inputs utilized or transfers between fair value measurement levels during the three and nine months ended September 30, 2018. The fair values of Level 2 instruments classified as cash equivalents, marketable debt securities and our marketable equity security investment in Ionis Pharmaceuticals, Inc. (Ionis) were determined through third-party pricing services. For additional information on our new agreement with Ionis, please read Note 17, Collaborative and Other Relationships, to these condensed consolidated financial statements. For a description of our validation procedures related to prices provided by third-party pricing services, please read Note 1, Summary of Significant Accounting Policies - Fair Value Measurements, to our consolidated financial statements included in our 2017 Form 10-K.

Debt Instruments

The fair and carrying values of our debt instruments, which are Level 2 liabilities, are summarized as follows:



	As of September 30, 2018				As of December 31, 2017
(In millions)	Fair Value		Carrying Value		Fair Value		Carrying Value
Notes payable to Fumedica AG	$	—	$	—	$	3.2	$	3.2
2.900% Senior Notes due September 15, 2020	1,491.6		1,476.2		1,517.7		1,482.4
3.625% Senior Notes due September 15, 2022	1,001.7		995.2		1,032.9		994.3
4.050% Senior Notes due September 15, 2025	1,756.6		1,737.4		1,851.9		1,736.3
5.200% Senior Notes due September 15, 2045	1,861.1		1,722.3		2,077.6		1,722.0
Total	$	6,111.0	$	5,931.1	$	6,483.3	$	5,938.2

In connection with our 2006 distribution agreement with Fumedica AG, we issued notes totaling 61.4 million Swiss Francs that were payable to Fumedica AG in varying amounts from June 2008 through June 2018. In June 2018 we redeemed our remaining note payable to Fumedica AG.

The fair value of our notes payable to Fumedica AG, as of December 31, 2017, was estimated using market observable inputs, including current interest and foreign currency exchange rates. The fair values of each of our series of Senior Notes were determined through market, observable and corroborated sources. For additional information on our debt instruments, please read Note 12, Indebtedness, to our consolidated financial statements included in our 2017 Form 10-K.

Contingent Consideration Obligations

In connection with our acquisitions of Convergence Pharmaceuticals Ltd., Stromedix Inc. (Stromedix) and Biogen International Neuroscience GmbH in 2015, 2012 and 2010, respectively, we agreed to make additional payments based upon the achievement of certain milestone events. The following table provides a roll forward of the fair values of our contingent consideration obligations, which includes Level 3 measurements:



	For the Three Months Ended September 30,					For the Nine Months Ended September 30,
(In millions)	2018			2017		2018			2017
Fair value, beginning of period	$	499.9		$	492.1	$	523.6		$	467.6
Changes in fair value	(87.9		)	30.0		(91.6		)	61.2
Payments	—			—		(20.0		)	(6.7		)
Fair value, end of period	$	412.0		$	522.1	$	412.0		$	522.1

As of September 30, 2018 and December 31, 2017, $330.5 million and $279.0 million, respectively, of the fair value of our contingent consideration obligations was reflected as a component of other long-term liabilities in our condensed consolidated balance sheets with the remaining balance reflected as a component of accrued expenses and other.

For the three and nine months ended September 30, 2018, changes in the fair value of our contingent consideration obligations were primarily due to lower cumulative probabilities of success related to our vixotrigine program for the treatment of TGN, an increase in interest rates used to revalue our contingent consideration liabilities and the passage of time. In addition, we dosed our first patient in the Phase 2b for BG00011 (STX-100) in September 2018 and expect to pay an $81.5 million milestone payment to former shareholders of Stromedix during the fourth quarter of 2018. For additional information on our IPR&D intangible asset related to our vixotrigine program for the treatment of TGN, please read Note 6, Intangible Assets and Goodwill, to these condensed consolidated financial statements.

For the three and nine months ended September 30, 2017, changes in the fair value of our contingent consideration obligations were primarily due to an increase in the probability of achieving certain developmental milestones based upon the progression of underlying clinical programs.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Financial Instruments

9 Months Ended

Sep. 30, 2018

Investments, Debt and Equity Securities [Abstract]

Financial Instruments

The following table summarizes our financial assets with maturities of less than 90 days from the date of purchase included in cash and cash equivalents in our condensed consolidated balance sheets:



(In millions)	As of September 30, 2018		As of December 31, 2017
Commercial paper	$	125.3	$	30.5
Overnight reverse repurchase agreements	23.7		3.6
Money market funds	1,695.1		948.0
Short-term debt securities	56.8		247.3
Total	$	1,900.9	$	1,229.4

The carrying values of our commercial paper, including accrued interest, overnight reverse repurchase agreements, money market funds and short-term debt securities approximate fair value due to their short-term maturities.

Upon the adoption of ASU 2016-01, our marketable equity securities gains (losses) are recorded in other income (expense), net in our condensed consolidated statements of income. The following tables summarize our marketable debt and equity securities, classified as available-for-sale:



As of September 30, 2018 (In millions)	Fair Value		Gross Unrealized Gains		Gross Unrealized Losses			Amortized Cost
Corporate debt securities
Current	$	1,273.7	$	0.1	$	(0.4	)	$	1,274.0
Non-current	677.9		0.5		(0.8		)	678.2
Government securities
Current	767.9		—		(0.5		)	768.4
Non-current	293.0		—		(0.6		)	293.6
Mortgage and other asset backed securities
Current	0.1		—		—			0.1
Non-current	273.6		0.1		(0.6		)	274.1
Total marketable debt securities	$	3,286.2	$	0.7	$	(2.9	)	$	3,288.4
Marketable equity securities, non-current	$	624.8	$	132.6	$	(4.4	)	$	496.6



As of December 31, 2017 (In millions)	Fair Value		Gross Unrealized Gains		Gross Unrealized Losses			Amortized Cost
Corporate debt securities
Current	$	1,039.3	$	—	$	(0.2	)	$	1,039.5
Non-current	1,570.5		0.9		—			1,569.6
Government securities
Current	1,075.1		0.1		(0.7		)	1,075.7
Non-current	844.2		0.2		(1.1		)	845.1
Mortgage and other asset backed securities
Current	0.8		—		—			0.8
Non-current	642.6		1.1		(0.8		)	642.3
Total marketable debt securities	$	5,172.5	$	2.3	$	(2.8	)	$	5,173.0
Marketable equity securities, non-current	$	11.8	$	1.8	$	(4.4	)	$	14.4

Summary of Contractual Maturities: Available-for-Sale Securities

The estimated fair value and amortized cost of our marketable debt securities available-for-sale by contractual maturity are summarized as follows:



	As of September 30, 2018				As of December 31, 2017
(In millions)	Estimated Fair Value		Amortized Cost		Estimated Fair Value		Amortized Cost
Due in one year or less	$	2,041.7	$	2,042.5	$	2,115.2	$	2,116.0
Due after one year through five years	1,111.2		1,112.3		2,730.0		2,730.0
Due after five years	133.3		133.6		327.3		327.0
Total available-for-sale securities	$	3,286.2	$	3,288.4	$	5,172.5	$	5,173.0

The average maturity of our marketable debt securities available-for-sale as of September 30, 2018 and December 31, 2017, was approximately 11 months and 17 months, respectively.

Proceeds from Marketable Debt Securities

The proceeds from maturities and sales of marketable debt securities and resulting realized gains and losses are summarized as follows:



	For the Three Months Ended September 30,						For the Nine Months Ended September 30,
(In millions)	2018			2017			2018			2017
Proceeds from maturities and sales	$	1,192.0		$	888.1		$	7,994.7		$	4,472.6
Realized gains	$	0.4		$	0.3		$	3.0		$	2.7
Realized losses	$	(0.6	)	$	(1.2	)	$	(10.8	)	$	(4.4	)

Strategic Investments

As of September 30, 2018 and December 31, 2017, our strategic investment portfolio was comprised of investments totaling $687.8 million and $85.8 million, respectively, which are included in investments and other assets in our condensed consolidated balance sheets. The increase in our strategic investment portfolio is a result of our investment in Ionis' common stock, as discussed below.

Our strategic investment portfolio includes investments in equity securities of certain biotechnology companies and venture capital funds where the underlying investments are in equity securities of certain biotechnology companies. Our investments in equity securities of certain publicly-traded biotechnology companies are regularly measured and carried at fair value and classified as Level 1 marketable equity securities within our disclosures included in Note 7, Fair Value Measurements, to these condensed consolidated financial statements. Our investment in Ionis' common stock will be regularly measured and carried at fair value and classified as a Level 2 marketable equity security within our disclosures in Note 7, Fair Value Measurements, to these condensed consolidated financial statements.

Ionis Pharmaceuticals, Inc.

In June 2018 we closed a new ten-year exclusive agreement with Ionis to develop novel antisense oligonucleotide drug candidates for a broad range of neurological diseases for a total payment of $1.0 billion consisting of an upfront payment of $375.0 million and the purchase of approximately 11.5 million shares of Ionis' common stock at a cost of $625.0 million.

Our investment in Ionis' common stock is remeasured each reporting period and carried at fair value. The effects of the holding period restrictions are estimated using an option pricing valuation model. The most significant assumptions within the model are the term of the restrictions and the stock price volatility, which is based upon historical volatility of similar companies. We also use a constant maturity risk-free interest rate to match the remaining term of the restrictions on our investment in Ionis' common stock and a dividend yield of zero based upon the fact that Ionis and similar companies generally have not historically granted cash dividends.

For additional information on our new agreement with Ionis, please read Note 17, Collaborative and Other Relationships, to these condensed consolidated financial statements.

Derivative Instruments

9 Months Ended

Sep. 30, 2018

Derivative Instruments and Hedging Activities Disclosure [Abstract]

Derivative Instruments

In August 2017 the FASB issued ASU 2017-12. We adopted this new standard on January 1, 2018, using the modified retrospective method, which did not have an impact on our financial position or results of operations; however, the adoption of this new standard resulted in additional disclosures and a change in the income statement classification with respect to where we recognize ineffective hedge transaction gains and losses. For additional information on this new standard, please read Note 1, Summary of Significant Accounting Policies - New Accounting Pronouncements, to these condensed consolidated financial statements.

Foreign Currency Forward Contracts - Hedging Instruments

Due to the global nature of our operations, portions of our revenues and operating expenses are recorded in currencies other than the U.S. dollar. The value of revenues and operating expenses measured in U.S. dollars is therefore subject to changes in foreign currency exchange rates. In order to mitigate these changes, we use foreign currency forward contracts to lock in exchange rates associated with a portion of our forecasted international revenues and operating expenses.

Foreign currency forward contracts in effect as of September 30, 2018 and December 31, 2017, had durations of 1 to 15 months and 1 to 21 months, respectively. These contracts have been designated as cash flow hedges and unrealized gains or losses on the portion of these foreign currency forward contracts that are included in the effectiveness test are reported in accumulated other comprehensive income (loss) (referred to as AOCI in the tables below). Realized gains and losses of such contracts are recognized in revenues when the sale of product in the currency being hedged is recognized and in operating expenses when the expense in the currency being hedged is recorded. Prior to the adoption of ASU 2017-12 on January 1, 2018, to the extent ineffective, hedge transaction gains and losses were reported in other income (expense), net. Effective January 1, 2018, we recognize all fair value changes of derivatives in earnings, including any ineffective portion, in the same line item in our condensed consolidated statements of income that has been impacted by the hedged item.

The notional value of foreign currency forward contracts that were entered into to hedge forecasted revenues and operating expenses is summarized as follows:



	Notional Amount
Foreign Currency: (In millions)	As of September 30, 2018		As of December 31, 2017
Euro	$	1,662.5	$	1,875.6
British pound	34.9		150.9
Canadian dollar	26.9		83.5
Swiss franc	9.4		88.7
Total foreign currency forward contracts	$	1,733.7	$	2,198.7

The pre-tax portion of the fair value of these foreign currency forward contracts that were included in accumulated other comprehensive income (loss) in total equity reflected net losses of $3.3 million and $113.0 million as of September 30, 2018 and December 31, 2017, respectively. We expect the net losses of $3.3 million to be settled over the next 15 months, of which $7.0 million of these losses are expected to be settled over the next 12 months, with any amounts in accumulated other comprehensive income (loss) to be reported as an adjustment to revenues or operating expenses. We consider the impact of our and our counterparties’ credit risk on the fair value of the contracts as well as the ability of each party to execute its contractual obligations. As of September 30, 2018 and December 31, 2017, credit risk did not change the fair value of our foreign currency forward contracts.

The following tables summarize the effect of foreign currency forward contracts designated as hedging instruments in our condensed consolidated statements of income:



For the Three Months Ended September 30, 2018							For the Nine Months Ended September 30, 2018
Net Gains/(Losses) Reclassified from AOCI into Operating Income (in millions)				Net Gains/(Losses) Recognized in Operating Income (in millions)			Net Gains/(Losses) Reclassified from AOCI into Operating Income (in millions)				Net Gains/(Losses) Recognized in Operating Income (in millions)
Location	2018			Location	2018		Location	2018			Location	2018
Revenues	$	(8.4	)	Revenues	$	0.3	Revenues	$	(51.7	)	Revenues	$	7.3
Operating expenses	$	(0.3	)	Operating expenses	$	0.4	Operating expenses	$	0.6		Operating expenses	$	—



For the Three Months Ended September 30, 2017							For the Nine Months Ended September 30, 2017
Net Gains/(Losses) Reclassified from AOCI into Operating Income (in millions)				Net Gains/(Losses) Recognized Directly into Net Income (in millions)			Net Gains/(Losses) Reclassified from AOCI into Operating Income (in millions)				Net Gains/(Losses) Recognized Directly into Net Income (in millions)
Location	2017			Location	2017		Location	2017			Location	2017
Revenues	$	(18.8	)	Other income (expense)	$	0.7	Revenue	$	(15.1	)	Other income (expense)	$	6.7
Operating expenses	$	0.5		Other income (expense)	$	0.2	Operating expenses	$	0.7		Other income (expense)	$	(0.1	)

Interest Rate Contracts - Hedging Instruments

We have entered into interest rate swap contracts on certain borrowing transactions to manage our exposure to interest rate changes.

In connection with the issuance of our 2.90% Senior Notes, we entered into interest rate swaps with an aggregate notional amount of $675.0 million, which expire on September 15, 2020. The interest rate swap contracts are designated as hedges of the fair value changes in our 2.90% Senior Notes attributable to changes in interest rates. Since the specific terms and notional amount of the swaps match the debt being hedged, these contracts are assumed to be highly effective and all changes in the fair value of the swaps are recognized as a component of our 2.90% Senior Notes with no net impact recorded in income. Any net interest payments made or received on the interest rate swap contracts are recognized as a component of interest expense in our condensed consolidated statements of income.

Foreign Currency Forward Contracts - Other Derivatives

We also enter into other foreign currency forward contracts, usually with durations of one month or less, to mitigate the foreign currency risk related to certain balance sheet positions. We have not elected hedge accounting for these transactions.

The aggregate notional amount of these outstanding foreign currency contracts was $691.7 million and $564.9 million as of September 30, 2018 and December 31, 2017, respectively. Net gains of $5.2 million and $4.8 million related to these contracts were recognized as a component of other income (expense), net for the three and nine months ended September 30, 2018, respectively, compared to net gains of $1.2 million and $5.7 million, respectively, in the prior year comparative periods.

Summary of Derivatives

While certain of our derivative instruments are subject to netting arrangements with our counterparties, we do not offset derivative assets and liabilities in our condensed consolidated balance sheets.

The following table summarizes the fair value and presentation in our condensed consolidated balance sheets of our outstanding derivative instruments, including those designated as hedging instruments:



		Fair Value
(In millions)	Balance Sheet Location	As of September 30, 2018
Hedging Instruments:
Asset derivatives	Other current assets	$	25.6
	Investments and other assets	$	5.4
Liability derivatives	Accrued expenses and other	$	10.9
	Other long-term liabilities	$	18.7
Other Derivatives:
Asset derivatives	Other current assets	$	4.1
Liability derivatives	Accrued expenses and other	$	1.6



		Fair Value
(In millions)	Balance Sheet Location	As of December 31, 2017
Hedging Instruments:
Asset derivatives	Other current assets	$	0.7
	Investments and other assets	$	0.2
Liability derivatives	Accrued expenses and other	$	84.7
	Other long-term liabilities	$	23.6
Other Derivatives:
Asset derivatives	Other current assets	$	1.8
Liability derivatives	Accrued expenses and other	$	3.0

Property, Plant and Equipment Property, Plant and Equipment	9 Months Ended
	Sep. 30, 2018
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment	Property, Plant and Equipment Property, plant and equipment are recorded at historical cost, net of accumulated depreciation. Accumulated depreciation on property, plant and equipment was $1,730.6 million and $1,559.1 million as of September 30, 2018 and December 31, 2017, respectively. Solothurn, Switzerland Facility We are building a large-scale biologics manufacturing facility in Solothurn, Switzerland. We expect this facility to be operational by the end of 2020. Upon completion, the facility will include 393,000 square feet related to a large-scale biologics manufacturing facility, 290,000 square feet of warehouse, utilities and support space and 51,000 square feet of administrative space. As of September 30, 2018 and December 31, 2017, we had approximately $1.6 billion and $1.2 billion, respectively, capitalized as construction in progress related to this facility. As of September 30, 2018, we had contractual commitments of approximately $200.0 million outstanding related to the construction of this facility.

Equity

9 Months Ended

Sep. 30, 2018

Equity [Abstract]

Equity

Total equity as of September 30, 2018, increased $1.2 billion compared to December 31, 2017. This increase was primarily due to net income attributable to Biogen Inc. of approximately $3.5 billion and a net cumulative-effect adjustment of approximately $0.5 billion recognized to retained earnings upon the adoptions of ASUs 2016-16 and 2016-01, partially offset by share repurchases totaling $3.0 billion, as described below.

For additional information on our adoption of ASUs 2016-16 and 2016-01, please read Note 1, Summary of Significant Accounting Policies - New Accounting Pronouncements, to these condensed consolidated financial statements.

Share Repurchases

In August 2018 our Board of Directors authorized a program to repurchase up to $3.5 billion of our common stock (2018 Share Repurchase Program). Our 2018 Share Repurchase Program does not have an expiration date. All share repurchases under our 2018 Share Repurchase Program will be retired. We did not repurchase any shares of our common stock under our 2018 Share Repurchase Program during the three and nine months ended September 30, 2018.

In July 2016 our Board of Directors authorized a program to repurchase up to $5.0 billion of our common stock (2016 Share Repurchase Program), which was completed as of June 30, 2018. All share repurchases under our 2016 Share Repurchase Program were retired. Under our 2016 Share Repurchase Program, we repurchased and retired 10.5 million shares of our common stock at a cost of $3.0 billion during the nine months ended September 30, 2018, and we repurchased and retired 3.7 million shares of our common stock at a cost of $1.0 billion during the nine months ended September 30, 2017. We did not repurchase any shares of our common stock under our 2016 Share Repurchase Program during the three months ended September 30, 2017.

In February 2011 our Board of Directors authorized a program to repurchase up to 20.0 million shares of our common stock (2011 Share Repurchase Program), which was completed as of March 31, 2017. Share repurchases under our 2011 Share Repurchase Program were principally used to offset common stock issuances under our share-based compensation programs. Under our 2011 Share Repurchase Program, we repurchased 1.2 million shares of our common stock at a cost of $365.4 million during the nine months ended September 30, 2017.

Noncontrolling Interests

The following table reconciles equity (deficit) attributable to noncontrolling interests (NCI):



	For the Three Months Ended September 30,						For the Nine Months Ended September 30,
(In millions)	2018			2017			2018			2017
NCI, beginning of period	$	(7.2	)	$	(11.6	)	$	(14.7	)	$	(11.5	)
Net income (loss) attributable to NCI, net of tax	(1.5		)	—			45.2			(0.1		)
Capital contribution by noncontrolling interest	2.0			—			13.1			—
Distribution to noncontrolling interest	—			—			(50.0		)	—
Translation adjustment and other	(0.1		)	—			(0.4		)	—
NCI, end of period	$	(6.8	)	$	(11.6	)	$	(6.8	)	$	(11.6	)

Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss)

9 Months Ended

Sep. 30, 2018

Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]

Accumulated Other Comprehensive Income (Loss)

The following table summarizes the changes in accumulated other comprehensive income (loss), net of tax by component:



(In millions)	Unrealized Gains (Losses) on Securities Available for Sale, Net of Tax			Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax			Unfunded Status of Postretirement Benefit Plans, Net of Tax			Translation Adjustments			Total
Balance, December 31, 2017	$	(1.6	)	$	(104.5	)	$	(36.8	)	$	(175.5	)	$	(318.4	)
Amount reclassified, net of tax, upon adoption of ASU 2016-01	1.5			—			—			—			1.5
Balance, January 1, 2018	(0.1		)	(104.5		)	(36.8		)	(175.5		)	(316.9		)
Other comprehensive income (loss) before reclassifications	(7.9		)	58.3			0.2			(38.8		)	11.8
Amounts reclassified from accumulated other comprehensive income (loss)	6.1			50.7			—			—			56.8
Net current period other comprehensive income (loss)	(1.8		)	109.0			0.2			(38.8		)	68.6
Balance, September 30, 2018	$	(1.9	)	$	4.5		$	(36.6	)	$	(214.3	)	$	(248.3	)



(In millions)	Unrealized Gains (Losses) on Securities Available for Sale, Net of Tax			Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax			Unfunded Status of Postretirement Benefit Plans, Net of Tax			Translation Adjustments			Total
Balance, December 31, 2016	$	(10.8	)	$	57.8		$	(32.7	)	$	(334.2	)	$	(319.9	)
Other comprehensive income (loss) before reclassifications	5.5			(176.5		)	(0.5		)	146.7			(24.8		)
Amounts reclassified from accumulated other comprehensive income (loss)	1.1			14.2			—			—			15.3
Net current period other comprehensive income (loss)	6.6			(162.3		)	(0.5		)	146.7			(9.5		)
Balance, September 30, 2017	$	(4.2	)	$	(104.5	)	$	(33.2	)	$	(187.5	)	$	(329.4	)

The following table summarizes the amounts reclassified from accumulated other comprehensive income:



(In millions)	Income Statement Location	Amounts Reclassified from Accumulated Other Comprehensive Income
		For the Three Months Ended September 30,						For the Nine Months Ended September 30,
		2018			2017			2018			2017
Gains (losses) on securities available for sale	Other income (expense)	$	(0.1	)	$	(0.9	)	$	(7.7	)	$	(1.7	)
	Income tax benefit (expense)	—			0.3			1.6			0.6

Gains (losses) on cash flow hedges	Revenues	(8.4		)	(18.8		)	(51.7		)	(15.1		)
	Operating expenses	(0.3		)	0.5			0.6			0.7
	Other income (expense)	0.1			0.1			0.2			0.2
	Income tax benefit (expense)	—			—			0.2			—

Total reclassifications, net of tax		$	(8.7	)	$	(18.8	)	$	(56.8	)	$	(15.3	)

Earnings per Share

9 Months Ended

Sep. 30, 2018

Earnings Per Share [Abstract]

Earnings per Share

Basic and diluted earnings per share are calculated as follows:



	For the Three Months Ended September 30,				For the Nine Months Ended September 30,
(In millions)	2018		2017		2018		2017
Numerator:
Net income attributable to Biogen Inc.	$	1,444.4	$	1,226.1	$	3,483.9	$	2,836.5
Denominator:
Weighted-average number of common shares outstanding	201.4		211.4		206.6		213.0
Effect of dilutive securities:
Stock options and employee stock purchase plan	—		0.1		—		—
Time-vested restricted stock units	0.4		0.2		0.3		0.2
Market stock units	0.1		0.1		0.1		0.1
Performance stock units settled in stock	—		—		—		—
Dilutive potential common shares	0.5		0.4		0.4		0.3
Shares used in calculating diluted earnings per share	201.9		211.8		207.0		213.3

Amounts excluded from the calculation of net income per diluted share because their effects were anti-dilutive were insignificant.

Share-based Payments

9 Months Ended

Sep. 30, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Share-based Payments

Share-based Compensation Expense

The following table summarizes share-based compensation expense included in our condensed consolidated statements of income:



	For the Three Months Ended September 30,						For the Nine Months Ended September 30,
(In millions)	2018			2017			2018			2017
Research and development	$	21.0		$	19.5		$	60.5		$	55.7
Selling, general and administrative	29.4			23.6			83.1			71.9
Subtotal	50.4			43.1			143.6			127.6
Capitalized share-based compensation costs	(3.5		)	(2.5		)	(9.7		)	(7.6		)
Share-based compensation expense included in total cost and expenses	46.9			40.6			133.9			120.0
Income tax effect	(7.7		)	(10.9		)	(21.8		)	(31.8		)
Share-based compensation expense included in net income attributable to Biogen Inc.	$	39.2		$	29.7		$	112.1		$	88.2

The following table summarizes share-based compensation expense associated with each of our share-based compensation programs:



	For the Three Months Ended September 30,						For the Nine Months Ended September 30,
(In millions)	2018			2017			2018			2017
Market stock units	$	7.4		$	4.5		$	20.6		$	17.1
Time-vested restricted stock units	29.9			26.7			96.6			81.2
Cash settled performance units	5.8			7.0			9.4			13.0
Performance units	3.0			3.2			4.2			8.9
Performance stock units settled in stock	1.3			—			3.4			—
Performance stock units settled in cash	1.1			—			1.5			—
Employee stock purchase plan	1.9			1.7			7.9			7.4
Subtotal	50.4			43.1			143.6			127.6
Capitalized share-based compensation costs	(3.5		)	(2.5		)	(9.7		)	(7.6		)
Share-based compensation expense included in total cost and expenses	$	46.9		$	40.6		$	133.9		$	120.0

We estimate the fair value of our obligations associated with our performance units, cash settled performance units and performance stock units settled in cash at the end of each reporting period through expected settlement. Cumulative adjustments to these obligations are recognized each quarter to reflect changes in the stock price and estimated outcome of the performance-related conditions.

Performance Stock Units (PSUs)

PSUs Settled in Stock

During the first quarter of 2018 we began granting awards for performance-vested restricted stock units that will settle in stock. PSUs awarded to employees have a three-year performance period and vest on the third anniversary of the grant date. The vesting of these awards is subject to the respective employee’s continued employment. The number of PSUs granted represents the target number of units that are eligible to be earned based upon the achievement of cumulative three-year performance measures established at the beginning of the performance period, which ends on December 31 of the third year of the performance period.

During the nine months ended September 30, 2018, 67,000 PSUs that will settle in stock were granted at a weighted average grant date fair value of $317.09.

PSUs Settled in Cash

During the first quarter of 2018 we began granting awards for performance-vested restricted stock units that will settle in cash. PSUs awarded to employees have three performance periods and vest on the third anniversary of the grant date. The vesting of these awards is subject to the respective employee’s continued employment. The number of PSUs granted represents the target number of units that are eligible to be earned based upon the achievement of three annual performance measures established when the performance objectives are defined, which will be at the beginning of each year and will end on December 31 of such year.

Participants may ultimately earn between 0% and 200% of the target number of PSUs granted based on the degree of achievement of the applicable performance metric. Accordingly, additional PSUs may be issued or currently outstanding PSUs may be cancelled upon final determination of the number of units earned. PSUs will be settled in cash based on the 30 calendar day average closing stock price through the vesting date, once the actual vested and earned number of PSUs is determined. Since no shares are issued, these awards do not dilute equity. Compensation expense, including the effect of forfeitures, is recognized over the applicable service period.

During the nine months ended September 30, 2018, 45,000 PSUs that will settle in cash were granted.

Income Taxes

9 Months Ended

Sep. 30, 2018

Income Tax Disclosure [Abstract]

Income Taxes

Tax Reform

The Tax Cuts and Jobs Act of 2017 (2017 Tax Act), which was signed into law in December 2017, has resulted in significant changes to the U.S. corporate income tax system. These changes include a federal statutory rate reduction from 35% to 21%, the elimination or reduction of certain domestic deductions and credits and limitations on the deductibility of interest expense and executive compensation. The 2017 Tax Act also transitions international taxation from a worldwide system to a modified territorial system and includes base erosion prevention measures on non-U.S. earnings, which has the effect of subjecting certain earnings of our foreign subsidiaries to U.S. taxation as global intangible low-taxed income (GILTI). These changes became effective in 2018. We do not recognize deferred taxes for basis differences expected to reverse as GILTI is incurred and instead account for any taxes assessed as period costs.

During the fourth quarter of 2017 we recognized within our provision for income taxes a $1.2 billion provisional estimate under the U.S. Securities and Exchange Commission Staff Accounting Bulletin No. 118. Our provisional estimate included an amount of $989.6 million resulting from a one-time mandatory deemed repatriation tax on accumulated foreign subsidiaries' previously untaxed foreign earnings (the Transition Toll Tax), as discussed below, and $184.0 million related to the impact of remeasuring our deferred tax balances to reflect the new federal statutory rate and other changes to U.S. tax law. During the three months ended September 30, 2018, we recognized a net reduction of $34.6 million in our estimated Transition Toll Tax, an expense of $5.1 million to remeasure our deferred tax balances and an $11.0 million expense to reflect other aspects of the 2017 Tax Act. During the nine months ended September 30, 2018, the remeasurement of our deferred tax balances resulted in an expense totaling $12.7 million.

Transition Toll Tax

The 2017 Tax Act eliminated the deferral of U.S. income tax on the historical unrepatriated earnings by imposing the Transition Toll Tax. The Transition Toll Tax was assessed on our share of our foreign corporations' accumulated foreign earnings that were not previously taxed. Earnings in the form of cash and cash equivalents were taxed at a rate of 15.5% and all other earnings were taxed at a rate of 8.0%.

At December 31, 2017, we considered none of our earnings to be permanently reinvested outside the U.S. and therefore recorded tax liabilities associated with an estimate of the total withholding taxes expected as a result of our repatriation of earnings. As a result, our estimate of the total withholding taxes may change as the amounts are finalized. As of September 30, 2018 and December 31, 2017, we have accrued income tax liabilities of $695.9 million and $989.6 million, respectively, under the Transition Toll Tax. The decrease in this liability is primarily attributed to our 2018 Transition Toll Tax payment of $85.0 million, the application by the U.S. Internal Revenue Service (IRS) of an approximately $150.0 million overpayment against the accrual and the impact of the $34.6 million adjustment described above. Of the amounts accrued as of September 30, 2018, no amounts are expected to be paid within one year based on our interpretation of how current year payments are applied. The Transition Toll Tax will be paid in installments over an eight-year period, which started in 2018, and will not accrue interest.

Status of our Assessment

The final determination of the Transition Toll Tax and remeasurement of our deferred assets and liabilities will be completed as additional information becomes available, but no later than one year from the enactment of the 2017 Tax Act.

Our preliminary estimate of the Transition Toll Tax and the remeasurement of our deferred tax assets and liabilities is subject to the finalization of management’s analysis related to certain matters, such as developing interpretations of the provisions of the 2017 Tax Act and changes to certain estimates and amounts related to the earnings and profits of certain subsidiaries. U.S. Treasury regulations, administrative interpretations or court decisions interpreting the 2017 Tax Act may require further adjustments and changes in our estimates.

For additional information on the 2017 Tax Act, please read Note 17, Income Taxes, to our consolidated financial statements included in our 2017 Form 10-K.

Tax Rate

A reconciliation between the U.S. federal statutory tax rate and our effective tax rate is summarized as follows:



	For the Three Months Ended September 30,				For the Nine Months Ended September 30,
	2018		2017		2018		2017
Statutory rate	21.0	%	35.0	%	21.0	%	35.0	%
State taxes	0.6		0.7		0.7		0.6
Taxes on foreign earnings	0.8		(11.4	)	0.1		(11.4	)
Credits and net operating loss utilization	(1.0	)	(0.7	)	(0.8	)	(0.8	)
Purchased intangible assets	0.3		1.2		0.5		1.3
Manufacturing deduction	—		(2.0	)	—		(2.1	)
Other permanent items	0.3		0.6		0.3		0.7
Tax reform	(0.6	)	—		(0.3	)	—
Other	(1.0	)	0.4		(0.2	)	0.6
Effective tax rate	20.4	%	23.8	%	21.3	%	23.9	%

Changes in Tax Rate

For the three and nine months ended September 30, 2018, compared to the same periods in 2017, the decreases in our effective tax rates were primarily due to the enactment of the 2017 Tax Act and lower 2018 estimated Branded Pharmaceutical Drug fee expense, which is not tax deductible. The effects of an overall reduction in the federal statutory rate in the U.S. were partially offset by the elimination of the manufacturing deduction, the imposition of the new GILTI tax on international earnings, limits on the deductibility of certain benefits and executive compensation and a reduction in the tax benefit associated with the Orphan Drug Credit, all resulting from the 2017 Tax Act, and a change in accounting rules related to recording the tax impacts of intercompany transactions. The effective tax rate for the nine months ended September 30, 2017, also reflected the impact of a favorable settlement related to a state tax matter in 2017.

Deferred Tax Assets and Liabilities

In addition to deferred tax assets and liabilities, we have recorded prepaid tax and deferred charges related to intercompany transactions. In October 2016 the FASB issued ASU 2016-16. This new standard eliminates the deferral of the tax effects of intra-entity asset transfers other than inventory. As a result, the income tax consequences from the intra-entity transfer of an asset other than inventory and associated changes to deferred taxes will be recognized when the transfer occurs.

We adopted this new standard on January 1, 2018, using the modified retrospective method, through a cumulative-effect adjustment to retained earnings as of that date. Upon adoption, we recognized additional deferred tax assets of approximately $2.0 billion offset by a corresponding increase to deferred tax liabilities of approximately $1.5 billion and an increase to retained earnings of approximately $0.5 billion. We will recognize incremental deferred income tax expense thereafter as these deferred tax assets and liabilities are utilized.

Accounting for Uncertainty in Income Taxes

We and our subsidiaries are routinely examined by various taxing authorities. We file income tax returns in various U.S. states and in U.S. federal and other foreign jurisdictions. With few exceptions, we are no longer subject to U.S. federal tax examination for years before 2013 or state, local or non-U.S. income tax examinations for years before 2010.

The IRS and other national tax authorities routinely examine our intercompany transfer pricing with respect to intellectual property related transactions and it is possible that they may disagree with one or more positions we have taken with respect to such valuations.

International Uncertain Tax Positions

We have made payments totaling approximately $60.0 million to the Danish Tax Authority (SKAT) for assessments received for 2009, 2011 and 2013 regarding withholding taxes and the treatment of certain intercompany transactions involving a Danish affiliate and another of our affiliates. We continue to dispute the assessments for all of these periods and believe that the positions taken in our historical filings are valid. It is reasonably possible that we will adjust the value of our uncertain tax positions related to Danish withholding taxes based on potential European court decisions expected in 2018 on similar matters.

Federal and State Uncertain Tax Positions

It is reasonably possible that we will adjust the value of our uncertain tax positions related to certain transfer pricing issues as we receive additional information from various taxing authorities, including reaching settlements with such authorities.

Other Consolidated Financial Statement Detail

9 Months Ended

Sep. 30, 2018

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Other Consolidated Financial Statement Detail

Other Income (Expense), Net

Components of other income (expense), net, are summarized as follows:



	For the Three Months Ended September 30,						For the Nine Months Ended September 30,
(In millions)	2018			2017			2018			2017
Interest income	$	26.0		$	20.6		$	81.4		$	54.2
Interest expense	(49.0		)	(61.8		)	(151.2		)	(188.8		)
Gain (loss) on investments, net	141.1			(4.0		)	132.0			(15.0		)
Foreign exchange gains (losses), net	0.2			6.7			(13.8		)	8.4
Other, net	(3.2		)	(5.5		)	(8.8		)	(9.4		)
Total other income (expense), net	$	115.1		$	(44.0	)	$	39.6		$	(150.6	)

For the three and nine months ended September 30, 2018, gain (loss) on investments, net, as reflected in the table above, substantially relate to marketable equity securities held at September 30, 2018.

Other Current Assets

Other current assets were $848.3 million and $962.0 million as of September 30, 2018 and December 31, 2017, and include prepaid taxes totaling $431.8 million and $657.6 million, respectively.

Accrued Expenses and Other

Accrued expenses and other consists of the following:



(In millions)	As of September 30, 2018		As of December 31, 2017
Revenue-related reserves for discounts and allowances	$	758.5	$	572.0
Current portion of contingent consideration obligations	381.5		844.6
Employee compensation and benefits	274.1		297.7
Royalties and licensing fees	226.3		206.7
Construction in progress	196.6		159.7
Collaboration expenses	101.5		183.7
Other	640.0		636.9
Total accrued expenses and other	$	2,578.5	$	2,901.3

Other Long-term Liabilities

Other long-term liabilities were $1,511.8 million and $1,628.7 million as of September 30, 2018 and December 31, 2017, and include accrued income taxes totaling $786.9 million and $979.8 million, respectively.

Collaborative and Other Relationships

9 Months Ended

Sep. 30, 2018

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Collaborative and Other Relationships

Ionis Pharmaceuticals, Inc.

In the second quarter of 2018 $50.9 million of the $375.0 million upfront payment was recorded as prepaid services in our condensed consolidated balance sheets and the remaining $324.1 million was recorded as research and development expense in our condensed consolidated statements of income. The amount recognized as prepaid services represented the value of the employee resources committed to the arrangement to provide research and discovery services over the term of the agreement.

The 11.5 million shares of Ionis' common stock were purchased at a premium to their fair value at the transaction closing date. The premium consisted of acquiring the shares at a price above the fair value based on the trailing 10-day weighted-average close price prior to entering into thia agreement in April 2018 and the effect of certain holding period restrictions. We recorded an asset of $462.9 million in investments and other assets in our condensed consolidated balance sheets reflecting the fair value of the common stock and a charge of $162.1 million to research and development expense in our condensed consolidated statements of income in the second quarter of 2018, reflecting the premium paid for the common stock.

Our investment in Ionis' common stock is remeasured each reporting period. Changes in the fair value of our investment in Ionis' common stock, including the effect of the holding period restrictions, are reflected in other income (expense), net in our condensed consolidated statements of income. For additional information on the fair value of our investment in Ionis' common stock, please read Note 8, Financial Instruments, to these condensed consolidated financial statements.

We have the option to license therapies arising out of this agreement and will be responsible for the development and commercialization of such therapies. We may pay development milestones to Ionis of up to $125.0 million or $270.0 million for each program, depending on the indication, as well as royalties on potential net commercial sales.

For information on our other collaboration arrangements with Ionis, please read Note 20, Collaborative and Other Relationships, to our consolidated financial statements included in our 2017 Form 10-K.

AbbVie Inc.

We have a collaboration agreement with AbbVie for the development and commercialization of ZINBRYTA, which was approved for the treatment of RMS in the U.S. in May 2016 and in the E.U. in July 2016. In March 2018 we and AbbVie announced the voluntary worldwide withdrawal of ZINBRYTA for RMS.

Under this agreement, we and AbbVie conducted ZINBRYTA co-promotion activities in the U.S., E.U. and Canadian territories (Collaboration Territory), where development and commercialization costs and profits were shared equally. Outside of the Collaboration Territory, we were solely responsible for development and commercialization of ZINBRYTA and paid a tiered royalty to AbbVie as a percentage of net sales in the low to high teens.

As a result of the voluntary worldwide withdrawal of ZINBRYTA, we recognized $2.4 million in inventory charges and $12.8 million in losses related to the termination of research and development contracts and clinical trials in our condensed consolidated statements of income, net of an expected AbbVie reimbursement in the first quarter of 2018.

Co-promotion Profits and Losses

In the U.S., for the three and nine months ended September 30, 2018, we recognized a net reduction in revenues of $0.7 million and $7.9 million, respectively, to reflect our share of an overall net loss within the collaboration, compared to $2.8 million and $12.6 million, respectively, in the prior year comparative periods. These results include the collaboration's estimate of future returns of product in the U.S.

In the E.U. and Canada, for the three and nine months ended September 30, 2018, we recognized net profit-sharing income of $0.2 million and $2.0 million, respectively, to reflect AbbVie's 50% sharing of the net collaboration losses, compared to net profit-sharing expense of $0.7 million and $2.0 million, respectively, in the prior year comparative periods to reflect AbbVie's 50% sharing of the net collaboration profits. These results include the collaboration's estimate of future returns of product in the E.U. and Canada.

For additional information on our collaboration arrangement with AbbVie, please read Note 20, Collaborative and Other Relationships, to our consolidated financial statements included in our 2017 Form 10-K.

Eisai Co., Ltd.

BAN2401 and Elenbecestat Collaboration

We have a collaboration agreement with Eisai Co., Ltd. (Eisai) to jointly develop and commercialize BAN2401, a monoclonal antibody that targets amyloid beta aggregates, and elenbecestat, a BACE inhibitor, two Eisai product candidates for the treatment of AD (the BAN2401 and Elenbecestat Collaboration). Eisai serves as the global operational and regulatory lead for both compounds with all costs, including research, development and sales and marketing expenses, shared equally by us and Eisai; and, if applicable, following marketing approval in major markets, such as the U.S., the E.U. and Japan, we and Eisai will co-promote BAN2401 and elenbecestat and share profits equally. In smaller markets, Eisai will distribute these products and pay us a royalty.

For the three and nine months ended September 30, 2018, sales and marketing expenses related to the BAN2401 and Elenbecestat Collaboration were immaterial.

A summary of development expenses related to the BAN2401 and Elenbecestat Collaboration is as follows:



	For the Three Months Ended September 30,				For the Nine Months Ended September 30,
(In millions)	2018		2017		2018		2017
Total development expense incurred by the collaboration related to the advancement of BAN2401 and Elenbecestat	$	64.9	$	38.3	$	176.0	$	105.0
Biogen's share of BAN2401 and Elenbecestat development expense reflected in research and development expense in our condensed consolidated statements of income	$	32.5	$	19.2	$	88.0	$	52.5

Aducanumab Collaboration Agreement

We also have a collaboration agreement with Eisai to jointly develop and commercialize aducanumab, our anti-amyloid beta antibody candidate for the treatment of AD (Aducanumab Collaboration Agreement). Under the Aducanumab Collaboration Agreement, we lead the on-going Phase 3 development of aducanumab.

For the period through March 31, 2018, we were responsible for 100% of development expense incurred by the collaboration for the advancement of aducanumab (aducanumab development expense). For the period April 1, 2018 through December 31, 2018, Eisai is reimbursing us for 15% of aducanumab development expense incurred and, beginning January 1, 2019, will reimburse us for 45% of aducanumab development expense incurred. Upon commercialization, both companies will co-promote aducanumab with a region-based profit split. Sales and marketing expense incurred before commercialization are shared in proportion to the same region-based profit split that will be utilized to co-promote aducanumab.

A summary of development and sales and marketing expenses related to the Aducanumab Collaboration Agreement is as follows:



	For the Three Months Ended September 30,				For the Nine Months Ended September 30,
(In millions)	2018		2017		2018		2017
Total aducanumab development expense	$	64.8	$	73.3	$	204.8	$	202.2
Biogen's share of aducanumab development expense reflected in research and development expense in our condensed consolidated statements of income	$	55.1	$	73.3	$	183.6	$	202.2

Total aducanumab sales and marketing expense incurred by the collaboration	$	12.1	$	5.1	$	33.3	$	14.9
Biogen's share of aducanumab sales and marketing expense reflected in selling, general and administrative expense our condensed consolidated statements of income	$	5.1	$	5.1	$	19.0	$	14.9

We and Eisai also co-promote AVONEX, TYSABRI and TECFIDERA in Japan in certain settings and Eisai distributes AVONEX, TYSABRI, TECFIDERA and PLEGRIDY in India and other Asia-Pacific markets, excluding China.

For additional information on our collaboration arrangements with Eisai, please read Note 20, Collaborative and Other Relationships, to our consolidated financial statements included in our 2017 Form 10-K.

Genentech

We have certain business and financial rights with respect to RITUXAN for the treatment of non-Hodgkin's lymphoma, CLL and other conditions, GAZYVA for the treatment of CLL and follicular lymphoma, OCREVUS for the treatment of PPMS and RMS and other potential anti-CD20 therapies pursuant to our collaboration arrangements with Genentech, a wholly-owned member of the Roche Group.

RITUXAN

Genentech and its affiliates are responsible for the worldwide manufacture of RITUXAN, as well as all development and commercialization activities as follows:

U.S.

We have co-exclusively licensed our rights to develop, commercialize and market RITUXAN in the U.S.

Canada

We have co-exclusively licensed our rights to develop, commercialize and market RITUXAN in Canada.

GAZYVA

The Roche Group and its sub-licensees maintain sole responsibility for the development, manufacture and commercialization of GAZYVA in the U.S. We recognize our share of the development and commercialization expenses of GAZYVA as a reduction of our share of pre-tax profits in revenues from anti-CD20 therapeutic programs.

OCREVUS

In March 2017 the FDA approved OCREVUS for the treatment of RMS and PPMS. Pursuant to the terms of our collaboration arrangements with Genentech, we receive a tiered royalty on U.S. net sales from 13.5% and increasing up to 24% if annual net sales exceed $900.0 million. There will be a 50% reduction to these royalties if a biosimilar to OCREVUS is approved in the U.S.

In addition, we receive a gross 3% royalty on net sales of OCREVUS outside the U.S., with the royalty period lasting 11 years from the first commercial sale of OCREVUS on a country-by-country basis. OCREVUS has been approved for treatment of RMS and PPMS in the E.U. and certain other countries.

The commercialization of OCREVUS does not impact the percentage of the co-promotion profits we receive for RITUXAN or GAZYVA. Genentech is solely responsible for development and commercialization of OCREVUS and funding future costs. OCREVUS royalty revenues were based on our estimates from third party and market research data of OCREVUS sales occurring during the corresponding period. Differences between actual and estimated royalty revenues will be adjusted for in the period in which they become known, which is expected to be the following quarter.

Revenues from Anti-CD20 Therapeutic Programs

Revenues from anti-CD20 therapeutic programs are summarized as follows:



	For the Three Months Ended September 30,				For the Nine Months Ended September 30,
(In millions)	2018		2017		2018		2017
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA	$	358.0	$	325.1	$	1,066.6	$	996.1
Other revenues from anti-CD20 therapeutic programs	153.7		81.4		378.7		148.1
Total revenues from anti-CD20 therapeutic programs	$	511.7	$	406.5	$	1,445.3	$	1,144.2

For additional information on our relationship with Genentech, please read Note 20, Collaborative and Other Relationships, to our consolidated financial statements included in our 2017 Form 10-K.

Samsung Bioepis

Joint Venture Agreement

In February 2012 we entered into a joint venture agreement with Samsung BioLogics, establishing an entity, Samsung Bioepis, to develop, manufacture and market biosimilar pharmaceuticals. As of September 30, 2018, our

ownership interest in Samsung Bioepis was approximately 5%, which reflects the effect of additional equity financings in which we did not participate. In June 2018 we exercised an option to increase our ownership percentage in Samsung Bioepis from approximately 5% to approximately 49.9%. The completion of this share purchase transaction is subject to certain regulatory closing conditions in multiple jurisdictions and is expected to close in the fourth quarter of 2018. Upon closing, we expect to pay approximately $700.0 million to Samsung BioLogics. The exact share purchase price will depend on the timing of the closing and foreign currency exchange rates at that time.

We recognize our share of the results of operations related to our investment in Samsung Bioepis under the equity method of accounting one quarter in arrears when the results of the entity become available, which is reflected as equity in loss of investee, net of tax in our condensed consolidated statements of income. During 2015, as our share of losses exceeded the carrying value of our initial investment, we suspended recognizing additional losses. We expect to recommence recognition of our share of Samsung Bioepis' income (losses) upon acquiring the additional interest in Samsung Bioepis.

Commercial Agreement

We reflect revenues on sales of BENEPALI and FLIXABI to third parties in product revenues, net in our condensed consolidated statements of income and record the related cost of revenues and sales and marketing expenses in our condensed consolidated statements of income to their respective line items when these costs are incurred.

In August 2017 the European Commission granted a marketing authorization in the E.U. for IMRALDI, an adalimumab biosimilar referencing HUMIRA. In April 2018 we and Samsung Bioepis entered into an agreement with AbbVie for the commercialization of IMRALDI. Under the terms of the agreement, AbbVie granted us and Samsung Bioepis patent licenses for the use and sale of IMRALDI in Europe, on a country-by-country basis, and we and Samsung Bioepis make royalty payments to AbbVie. In October 2018 we began to recognize revenues on sales of IMRALDI to third parties in the E.U.

We share 50% of the profit or loss related to our commercial agreement with Samsung Bioepis, which is recognized in collaboration profit (loss) sharing in our condensed consolidated statements of income. For the three and nine months ended September 30, 2018, we recognized net profit-sharing expense of $47.7 million and $131.2 million, respectively, to reflect Samsung Bioepis' 50% sharing of the net collaboration profits, compared to $34.5 million and $80.5 million, respectively, in the prior year comparative periods.

Other Services

Simultaneous with the formation of Samsung Bioepis, we also entered into a license agreement, a technical development services agreement and a manufacturing agreement with Samsung Bioepis. For the three and nine months ended September 30, 2018, we recognized $48.1 million and $80.7 million, respectively, in relation to these services in other revenues in our condensed consolidated statements of income, compared to $8.8 million and $23.7 million, respectively, in the prior year comparative periods.

For additional information on our collaboration arrangement with Samsung Bioepis and our other significant collaboration arrangements, please read Note 20, Collaborative and Other Relationships, to our consolidated financial statements included in our 2017 Form 10-K.

Investments in Variable Interest Entities	9 Months Ended
Investments in Variable Interest Entities	Sep. 30, 2018
Investments in Variable Interest Entities [Abstract]
Investments in Variable Interest Entities	Investments in Variable Interest Entities Consolidated Variable Interest Entities Our condensed consolidated financial statements include the financial results of variable interest entities in which we are the primary beneficiary. The following are our significant variable interest entities. Neurimmune SubOne AG In November 2007 we entered into a collaboration and license agreement with Neurimmune SubOne AG (Neurimmune) for the development and commercialization of antibodies for the treatment of AD. We are responsible for the development, manufacturing and commercialization of all collaboration products. This agreement is effective for the longer of the duration of certain patents relating to a licensed product or 12 years from the first commercial sale of any product using such a licensed compound. Our anti-amyloid beta antibody candidate, aducanumab, for the treatment of AD resulted from this collaboration. We consolidate the results of Neurimmune as we determined that we are the primary beneficiary of Neurimmune because we have the power through the collaboration to direct the activities that most significantly impact the entity’s economic performance and we are required to fund 100% of the research and development costs incurred in support of the collaboration. Under this agreement, we are also required to pay royalties on net sales of any resulting commercial products and make payments upon the achievement of certain milestone events. In October 2017 we amended the terms of our collaboration and license agreement with Neurimmune. Under the amended agreement, we made a $150.0 million payment to Neurimmune in exchange for a 15% reduction in royalty rates payable on products developed under this agreement, including on potential commercial sales of aducanumab. In May 2018 we made an additional $50.0 million payment to Neurimmune to further reduce the previously negotiated royalty rates payable on products developed under this agreement, including on potential commercial sales of aducanumab, by an additional 5%. Our royalty rates payable on products developed under the amended agreement, including on potential commercial sales of aducanumab, will now range from the high single digits to low teens. As we consolidate the results of Neurimmune, we treated these payments as distributions and recognized them as charges to noncontrolling interest in the fourth quarter of 2017 and the second quarter of 2018, as applicable. Research and development costs for which we reimburse Neurimmune are reflected in research and development expense in our condensed consolidated statements of income. During the three and nine months ended September 30, 2018 and 2017, amounts reimbursed were immaterial. The assets and liabilities of Neurimmune are not significant to our condensed consolidated financial position or results of operations as it is a research and development organization. We have provided no financing to Neurimmune other than previously contractually required amounts. Under the terms of the Aducanumab Collaboration Agreement, Eisai had an option to share in the benefit and cost associated with the royalty reductions discussed above; however, Eisai elected to not share in the benefit and cost with respect to either the October 2017 or May 2018 royalty reductions, which will impact the amount of profits (losses) on potential commercial sales of aducanumab to be shared with Eisai. For additional information on our collaboration arrangements with Eisai, please read Note 17, Collaborative and Other Relationships, to these condensed consolidated financial statements. Unconsolidated Variable Interest Entities We have relationships with other variable interest entities that we do not consolidate as we lack the power to direct the activities that significantly impact the economic success of these entities. These relationships include investments in certain biotechnology companies and research collaboration agreements. As of September 30, 2018 and December 31, 2017, the carrying value of our investments in certain biotechnology companies representing unconsolidated variable interest entities totaled $29.2 million and $48.3 million, respectively. Our maximum exposure to loss related to these variable interest entities is limited to the carrying value of our investments. We have also entered into research collaboration agreements with certain variable interest entities where we are required to fund certain development activities. These development activities are included in research and development expense in our condensed consolidated statements of income as they are incurred. We have provided no financing to these variable interest entities other than previously contractually required amounts. For additional information on our investments in Neurimmune and other variable interest entities, please read Note 19, Investments in Variable Interest Entities, to our consolidated financial statements included in our 2017 Form 10-K.

Litigation	9 Months Ended
Litigation	Sep. 30, 2018
Loss Contingency, Information about Litigation Matters [Abstract]
Litigation	Litigation We are currently involved in various claims and legal proceedings, including the matters described below. For information as to our accounting policies relating to claims and legal proceedings, including use of estimates and contingencies, please read Note 1, Summary of Significant Accounting Policies, to our consolidated financial statements included in our 2017 Form 10-K. With respect to some loss contingencies, an estimate of the possible loss or range of loss cannot be made until management has further information, including, for example, (i) which claims, if any, will survive dispositive motion practice; (ii) information to be obtained through discovery; (iii) information as to the parties' damages claims and supporting evidence; (iv) the parties’ legal theories; and (v) the parties' settlement positions. The claims and legal proceedings in which we are involved also include challenges to the scope, validity or enforceability of the patents relating to our products, pipeline or processes and challenges to the scope, validity or enforceability of the patents held by others. These include claims by third parties that we infringe their patents. An adverse outcome in any of these proceedings could result in one or more of the following and have a material impact on our business or consolidated results of operations and financial position: (i) loss of patent protection; (ii) inability to continue to engage in certain activities; and (iii) payment of significant damages, royalties, penalties and/or license fees to third parties. Loss Contingencies Qui Tam Litigation In July 2015 a qui tam action filed by Michael Bawduniak on behalf of the U.S. and certain states was unsealed by the U.S. District Court for the District of Massachusetts. The action alleges sales and promotional activities in violation of the federal False Claims Act and state law counterparts and seeks single and treble damages, civil penalties, interest, attorneys’ fees and costs. Our motion to dismiss was denied in part. No trial date has been set. The U.S. has not made an intervention decision. An estimate of the possible loss or range of loss cannot be made at this time. In May 2018 we were served with a qui tam action filed by SMSF, LLC on behalf of the U.S. and certain states in the U.S. District Court for the District of Massachusetts. The case was filed under seal in July 2016 and unsealed in March 2018 after the U.S. declined to intervene. The case alleges activities by nurse-educators in violation of the federal False Claims Act and state law counterparts and seeks single and treble damages, civil penalties, interest, attorneys' fees and costs. No trial date has been set. An estimate of the possible loss or range of loss cannot be made at this time. In July 2018 we and certain other drug manufacturers and pharmacy benefit managers were served with a qui tam action filed by John Borzilleri on behalf of the U.S. and certain states in the U.S. District Court for the District of Rhode Island. The case was filed under seal in January 2014 and unsealed in April 2018 after the U.S. declined to intervene. The case alleges agreements with pharmacy benefit managers in violation of the False Claims Act and state law counterparts and seeks single and treble damages, civil penalties, interest, attorneys' fees and costs. No trial date has been set. An estimate of the possible loss or range of loss cannot be made at this time. Securities Litigation We and certain current and former officers are defendants in an action filed by a shareholder in October 2016 in the U.S. District Court for the District of Massachusetts alleging violations of federal securities laws under 15 U.S.C §78j(b) and §78t(a) and 17 C.F.R. §240.10b-5 and seeking a declaration of the action as a class action and an award of damages, interest and attorneys' fees. In March 2018 the court dismissed the complaint with prejudice. The plaintiff's appeal is pending. An estimate of the possible loss or range of loss cannot be made at this time. Other Matters Hatch-Waxman Act Litigation relating to TECFIDERA Orange-Book Listed Patents In June, July and September 2017 and in January, March and April 2018 we initiated patent infringement proceedings against multiple parties pursuant to the Hatch-Waxman Act in the U.S. District Courts for the District of Delaware, the Central District of California, the District of New Jersey, the Middle District of North Carolina, the Southern District of New York, the District of Colorado and the Northern District of West Virginia. The cases filed against Teva Pharmaceuticals USA, Inc., Banner Life Sciences LLC, Impax Laboratories Inc. (now known as Impax Laboratories LLC) and Par Pharmaceutical Inc. have been dismissed and the cases in all courts other than the U.S. District Courts for the District of Delaware and the Northern District of West Virginia have been dismissed. Patent infringement proceedings pursuant to the Hatch-Waxman Act are now pending against Amneal Pharmaceuticals LLC, Aurobindo Pharma U.S.A., Inc., Caribe Holdings (Cayman) Co. Ltd., DBA Puracap Caribe, Graviti Pharmaceuticals Pvt. Ltd., Hetero USA, Inc., Prinston Pharmaceutical Inc., Slayback Pharma LLC, Alkem Laboratories Ltd., Cipla Limited, Glenmark Pharmaceuticals Ltd., Lupin Atlantis Holdings SA, Macleods Pharmaceuticals, Ltd., MSN Laboratories Pvt. Ltd., Pharmathen S.A., Shilpa Medicare Limited, Sun Pharma Global FZE, Torrent Pharmaceuticals Ltd., TWi Pharmaceuticals, Inc., Windlas Healthcare Pvt. Ltd., Accord Healthcare Inc., Sandoz Inc., Sawai USA, Inc. and Zydus Pharmaceuticals (USA) Inc. in the U.S. District Court for the District of Delaware and against Mylan Pharmaceuticals Inc. in the U.S. District Court for the Northern District of West Virginia. A trial has been set for December 2019 in the Delaware actions, and a trial has been set for February 2020 in the West Virginia action. Petition for Inter Partes Review filed by Mylan Pharmaceuticals, Inc. In July 2018 Mylan Pharmaceuticals, Inc. filed a petition with the U.S. Patent Trial and Appeal Board seeking inter partes review of our U.S. Patent No. 8,399,514 (the '514 Patent). The '514 Patent includes claims covering the treatment of MS with 480 mg of dimethyl fumarate per day as provided for in our TECFIDERA label. The petition is pending. Interference Proceeding with Forward Pharma In April 2015 the U.S. Patent and Trademark Office (USPTO) declared an interference between Forward Pharma’s pending U.S. Patent Application No. 11/576,871 and the '514 Patent. In March 2017 the USPTO ruled against Forward Pharma. Forward Pharma has appealed to the U.S. Court of Appeals for the Federal Circuit and the appeal is pending. For additional information on this matter, please read Note 6, Intangibles Assets and Goodwill, to these condensed consolidated financial statements. European Patent Office Oppositions In 2016 the EPO revoked our European patent number 2 137 537 (the '537 Patent). We have appealed to the Technical Board of Appeal of the EPO and the appeal is pending. The '537 Patent includes claims covering the treatment of MS with 480 mg of dimethyl fumarate as provided for in our TECFIDERA label. In March 2018 the EPO revoked Forward Pharma’s European Patent No. 2 801 355, which was issued in May 2015 and expires in October 2025. Forward Pharma has filed an appeal to the Technical Board of Appeal of the EPO and the appeal is pending. The settlement and license agreement that we entered with Forward Pharma in January 2017 did not resolve the issues pending in this proceeding and we and Forward Pharma intend to permit the Technical Board of Appeal and the Enlarged Board of Appeal, if applicable, to make a final determination. For additional information on this matter, please read Note 6, Intangibles Assets and Goodwill, to these condensed consolidated financial statements. TYSABRI Patent Revocation Matters In November 2017 Bioeq GMBH, affiliated with the Polpharma Group, brought an action in the Polish Patent Office seeking to revoke Polish Patent Number 215263 (the Polish ‘263 Patent), the Polish patent corresponding to our European Patent Number 1 485 127 (the EU ‘127 Patent) (“Administration of agents to treat inflammation”). The Polish ‘263 Patent concerns administration of natalizumab (TYSABRI) to treat MS. The Polish ‘263 Patent was issued in 2013 and expires in February 2023. Swiss Pharma International AG, also affiliated with the Polpharma Group, filed actions in the District Court of The Hague (January 2016), the German Patents Court (March 2016) and the Commercial Court of Rome (November 2017) seeking to invalidate the Dutch, German and Italian counterparts of the EU '127 Patent, which was issued in 2011 and also concerns administration of natalizumab (TYSABRI) to treat MS. The EU '127 Patent expires in February 2023. The Dutch and German counterparts were ruled invalid and we have appealed. No date for a hearing on the merits has been set in the Polish and Italian actions. IMRALDI Patent Litigation In September 2018 Fresenius Kabi Deutschland GmbH (Fresenius Kabi) commenced proceedings for damages and injunctive relief against Biogen France SAS in the Tribunal de Grande Instance de Paris, alleging that IMRALDI, the biosimilar adalimumab product of Samsung Bioepis UK Ltd. that Biogen commercializes in Europe, infringes the French counterpart of European Patent No. 3 148 510 (the ‘510 Patent), which was issued in June 2018 and expires in May 2035. In October 2018 Fresenius Kabi commenced preliminary injunction proceedings against Biogen (Denmark) Manufacturing ApS and Biogen Denmark A/S in Denmark's Maritime and Commercial High Court, alleging infringement of the Danish counterpart of the '510 Patent. In August 2018 Biogen Idec Ltd. (Biogen UK) and Samsung Bioepis UK Ltd. filed an action in the United Kingdom Patents Court to revoke the U.K. counterpart to the ‘510 Patent. Fresenius Kabi has filed a counterclaim asserting infringement of the ‘510 Patent and seeking damages and an injunction to restrain infringement if the patent is found valid and infringed. A trial has been set for July 2019. '755 Patent Litigation In May 2010 Biogen MA Inc. (formerly Biogen Idec MA Inc.) filed a complaint in the U.S. District Court for the District of New Jersey alleging infringement by Bayer Healthcare Pharmaceuticals Inc. (Bayer) (manufacturer, marketer and seller of BETASERON and manufacturer of EXTAVIA), EMD Serono, Inc. (EMD Serono) (manufacturer, marketer and seller of REBIF), Pfizer (co-marketer of REBIF) and Novartis Pharmaceuticals Corp. (Novartis) (marketer and seller of EXTAVIA) of our U.S. Patent No. 7,588,755 ('755 Patent), which claims the use of interferon beta for immunomodulation or treating a viral condition, viral disease, cancers or tumors. The complaint seeks monetary damages, including lost profits and royalties. Bayer had previously filed a complaint against us in the same court, on May 27, 2010, seeking a declaratory judgment that it does not infringe the '755 Patent and that the '755 Patent is invalid, and seeking monetary relief in the form of attorneys' fees, costs and expenses. Bayer, Pfizer, Novartis and EMD Serono have all filed counterclaims seeking declaratory judgments of patent invalidity and non-infringement, and seeking monetary relief in the form of costs and attorneys' fees. In September 2018, following a trial against EMD Serono and Pfizer, the court granted Biogen's motion for judgment as a matter of law that the '755 Patent is infringed and valid and ordered a new trial on all damages issues. The court has not yet scheduled the new damages trial or a trial against Bayer and Novartis. Government Matters We have learned that state and federal governmental authorities are investigating our sales and promotional practices and have received related subpoenas. We are cooperating with the government. We have received subpoenas and other requests from the federal government for documents and information relating to our relationship with non-profit organizations that assist patients taking drugs sold by Biogen and Biogen's co-pay assistance programs. We are cooperating with the government. In July 2016 we received civil investigative demands from the federal government for documents and information relating to our treatment of certain service agreements with wholesalers when calculating and reporting Average Manufacturer Prices in connection with the Medicaid Drug Rebate Program. We are cooperating with the government. In July 2017 we learned that the Prosecution Office of Milan is investigating our interactions with certain healthcare providers in Italy. We are cooperating with the government. Tax Matter In the second quarter of 2018 the State Treasury of Goias, Brazil issued tax assessments for the period 2013 through February 2018 relating to tax on the circulation of goods and totaling approximately $70.0 million including interest and penalties. We dispute the assessments and have filed defenses with the Administrative Court of Appeals for the State of Goias, which are pending. We have not formed an opinion that an unfavorable outcome of the dispute is either probable or remote. Product Liability and Other Legal Proceedings We are also involved in product liability claims and other legal proceedings generally incidental to our normal business activities. While the outcome of any of these proceedings cannot be accurately predicted, we do not believe the ultimate resolution of any of these existing matters would have a material adverse effect on our business or financial condition.

Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Policies)

9 Months Ended

Sep. 30, 2018

Accounting Policies [Abstract]

Overview

Basis of presentation

We operate as one operating segment, focused on discovering, developing and delivering worldwide innovative therapies for people living with serious neurological and neurodegenerative diseases.

Consolidation

Use of estimates

New accounting pronouncements

Revenue Recognition

Product Revenues

Reserves for Discounts and Allowances

Product revenue reserves, which are classified as a reduction in product revenues, are generally characterized in the following categories: discounts, contractual adjustments and returns.

For additional information on our revenues, please read Note 4, Revenues, to these condensed consolidated financial statements.

Revenues from Anti-CD20 Therapeutic Programs

Revenues from anti-CD20 therapeutic programs consist of:


(i)	our share of pre-tax profits and losses in the U.S. for RITUXAN and GAZYVA; and


(ii)	other revenues from anti-CD20 therapeutic programs, which primarily consist of our share of pre-tax co-promotion profits on RITUXAN in Canada and royalty revenues on sales of OCREVUS.

For additional information on our relationship with Genentech, please read Note 20, Collaborative and Other Relationships, to our consolidated financial statements included in our 2017 Form 10-K.

Collaborative and Other Relationships

Royalty Revenues

Other Corporate Revenues

Accounts Receivable

We do not adjust our receivables for the effects of a significant financing component at contract inception if we expect to collect the receivables in one year or less from the time of sale.

The adoption of the new revenue standards did not change our historical accounting methods for our accounts receivable.

Financial Instruments

Leasing


•	ASU No. 2018-10, Codification Improvements to Topic 842, Leases, which amends certain narrow aspects of the guidance issued in ASU 2016-02.


•	ASU No. 2018-11, Leases (Topic 842): Targeted Improvements, which allows for a transition approach to initially apply ASU 2016-02 at the adoption date and recognize a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption as well as an additional practical expedient for lessors to not separate non-lease components from the associated lease component.

We will adopt the new leasing standards using a modified retrospective transition approach to be applied to leases existing as of, or entered into after, January 1, 2019.

Income Taxes

For additional information on our income taxes, please read Note 15, Income Taxes, to these condensed consolidated financial statements.

Net Periodic Pension Cost

Debt Securities

Derivative Instruments and Hedging Activities

For additional information on our derivative instruments and hedging activities, please read Note 9, Derivative Instruments, to these condensed consolidated financial statements.

Fair Value Measurements

Revenues Revenues (Tables)

9 Months Ended

Sep. 30, 2018

Revenue from Contract with Customer [Abstract]

Revenues by product

Revenues by product are summarized as follows:



	For the Three Months Ended September 30,
(In millions)	2018						2017
	United States		Rest of World		Total		United States		Rest of World		Total
Multiple Sclerosis:
TECFIDERA	$	842.1	$	247.9	$	1,090.0	$	836.3	$	233.3	$	1,069.6
Interferon*	421.5		168.6		590.1		473.3		188.7		662.0
TYSABRI	253.0		217.2		470.2		266.8		202.6		469.4
FAMPYRA	—		22.5		22.5		—		24.3		24.3
ZINBRYTA	—		—		—		—		14.2		14.2
Spinal Muscular Atrophy:
SPINRAZA	223.9		243.8		467.7		197.6		73.3		270.9
Other Product Revenues:
FUMADERM	—		4.8		4.8		—		10.7		10.7
BENEPALI	—		123.4		123.4		—		99.2		99.2
FLIXABI	—		11.4		11.4		—		2.2		2.2
Total product revenues	$	1,740.5	$	1,039.6	$	2,780.1	$	1,774.0	$	848.5	$	2,622.5

*Interferon includes AVONEX and PLEGRIDY.



	For the Nine Months Ended September 30,
(In millions)	2018						2017
	United States		Rest of World		Total		United States		Rest of World		Total
Multiple Sclerosis:
TECFIDERA	$	2,396.8	$	766.9	$	3,163.7	$	2,462.4	$	676.0	$	3,138.4
Interferon*	1,237.5		528.4		1,765.9		1,439.8		561.1		2,000.9
TYSABRI	768.2		631.3		1,399.5		861.7		648.7		1,510.4
FAMPYRA	—		69.9		69.9		—		67.4		67.4
ZINBRYTA	—		1.4		1.4		—		41.0		41.0
Spinal Muscular Atrophy:
SPINRAZA	617.8		636.5		1,254.3		438.8		82.4		521.2
Hemophilia:
ELOCTATE	—		—		—		42.2		6.2		48.4
ALPROLIX	—		—		—		21.0		5.0		26.0
Other Product Revenues:
FUMADERM	—		17.3		17.3		—		30.7		30.7
BENEPALI	—		359.9		359.9		—		253.2		253.2
FLIXABI	—		29.2		29.2		—		4.7		4.7
Total product revenues	$	5,020.3	$	3,040.8	$	8,061.1	$	5,265.9	$	2,376.4	$	7,642.3

*Interferon includes AVONEX and PLEGRIDY.

Analysis of change In reserves

An analysis of the change in reserves for discounts and allowances is summarized as follows:



(In millions)	Discounts			Contractual Adjustments			Returns			Total
Balance, as of December 31, 2017	$	109.6		$	606.0		$	46.0		$	761.6
Current provisions relating to sales in current year	498.9			1,949.6			18.0			2,466.5
Adjustments relating to prior years	(0.3		)	(7.0		)	0.1			(7.2		)
Payments/credits relating to sales in current year	(373.7		)	(1,285.8		)	(0.8		)	(1,660.3		)
Payments/credits relating to sales in prior years	(107.8		)	(495.2		)	(21.8		)	(624.8		)
Balance, as of September 30, 2018	$	126.7		$	767.6		$	41.5		$	935.8

Total reserves included in consolidated balance sheets

The total reserves above, which are included in our condensed consolidated balance sheets, are summarized as follows:



(In millions)	As of September 30, 2018		As of December 31, 2017
Reduction of accounts receivable	$	177.3	$	189.6
Component of accrued expenses and other	758.5		572.0
Total reserves	$	935.8	$	761.6

Revenues from anti-CD20 therapeutic programs

Revenues from anti-CD20 therapeutic programs are summarized as follows:



	For the Three Months Ended September 30,				For the Nine Months Ended September 30,
(In millions)	2018		2017		2018		2017
Biogen’s share of pre-tax profits in the U.S. for RITUXAN and GAZYVA	$	358.0	$	325.1	$	1,066.6	$	996.1
Other revenues from anti-CD20 therapeutic programs	153.7		81.4		378.7		148.1
Total revenues from anti-CD20 therapeutic programs	$	511.7	$	406.5	$	1,445.3	$	1,144.2

Other revenues

Other revenues are summarized as follows:



	For the Three Months Ended September 30,						For the Nine Months Ended September 30,
(In millions)	2018			2017			2018			2017
Revenues from collaborative and other relationships:
AbbVie	$	(0.7	)	$	(2.8	)	$	(7.9	)	$	(12.6	)
Samsung Bioepis and other	48.1			9.0			80.7			34.1
Other royalty and corporate revenues:
Royalty	7.9			11.8			35.8			49.1
Other corporate	91.9			30.8			311.6			109.8
Total other revenues	$	147.2		$	48.8		$	420.2		$	180.4

Inventory (Tables)

9 Months Ended

Sep. 30, 2018

Inventory Disclosure [Abstract]

Components of inventory

The components of inventory are summarized as follows:



(In millions)	As of September 30, 2018		As of December 31, 2017
Raw materials	$	181.8	$	162.4
Work in process	608.0		605.7
Finished goods	136.5		157.4
Total inventory	$	926.3	$	925.5

Balance Sheet Classification:
Inventory	$	916.6	$	902.7
Investments and other assets	9.7		22.8
Total inventory	$	926.3	$	925.5

Intangible Assets and Goodwill (Tables)

9 Months Ended

Sep. 30, 2018

Goodwill and Intangible Assets Disclosure [Abstract]

Intangible assets

Intangible assets, net of accumulated amortization, impairment charges and adjustments, are summarized as follows:



		As of September 30, 2018							As of December 31, 2017
(In millions)	Estimated Life	Cost		Accumulated Amortization			Net		Cost		Accumulated Amortization			Net
Out-licensed patents	13-23 years	$	543.3	$	(541.6	)	$	1.7	$	543.3	$	(535.6	)	$	7.7
Developed technology	15-23 years	3,005.3		(2,724.0		)	281.3		3,005.3		(2,689.0		)	316.3
In-process research and development	Indefinite until commercialization	480.9		—			480.9		680.6		—			680.6
Trademarks and tradenames	Indefinite	64.0		—			64.0		64.0		—			64.0
Acquired and in-licensed rights and patents	4-18 years	3,974.2		(1,423.1		)	2,551.1		3,971.4		(1,160.4		)	2,811.0
Total intangible assets		$	8,067.7	$	(4,688.7	)	$	3,379.0	$	8,264.6	$	(4,385.0	)	$	3,879.6

Estimated future amortization for acquired intangible assets

Based upon this analysis, the estimated future amortization of acquired intangible assets for the next five years is expected to be as follows:



(In millions)	As of September 30, 2018
2018 (remaining three months)	$	91.7
2019	367.0
2020	366.8
2021	249.4
2022	250.5
2023	230.0

Summary of roll forward of the changes in goodwill

The following table provides a roll forward of the changes in our goodwill balance:



(In millions)	As of September 30, 2018
Goodwill, beginning of period	$	4,632.5
Increase to goodwill	810.7
Other	(3.1		)
Goodwill, end of period	$	5,440.1

Fair Value Measurements (Tables)

9 Months Ended

Sep. 30, 2018

Fair Value Disclosures [Abstract]

Summary of assets and liabilities recorded at fair value



As of September 30, 2018 (In millions)	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	$	1,900.9	$	—	$	1,900.9	$	—
Marketable debt securities:
Corporate debt securities	1,951.6		—		1,951.6		—
Government securities	1,060.9		—		1,060.9		—
Mortgage and other asset backed securities	273.7		—		273.7		—
Marketable equity securities	624.8		95.6		529.2		—
Derivative contracts	35.1		—		35.1		—
Plan assets for deferred compensation	26.4		—		26.4		—
Total	$	5,873.4	$	95.6	$	5,777.8	$	—
Liabilities:
Derivative contracts	$	31.2	$	—	$	31.2	$	—
Contingent consideration obligations	412.0		—		—		412.0
Total	$	443.2	$	—	$	31.2	$	412.0



As of December 31, 2017 (In millions)	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	$	1,229.4	$	—	$	1,229.4	$	—
Marketable debt securities:
Corporate debt securities	2,609.8		—		2,609.8		—
Government securities	1,919.3		—		1,919.3		—
Mortgage and other asset backed securities	643.4		—		643.4		—
Marketable equity securities	11.8		11.8		—		—
Derivative contracts	2.7		—		2.7		—
Plan assets for deferred compensation	28.5		—		28.5		—
Total	$	6,444.9	$	11.8	$	6,433.1	$	—
Liabilities:
Derivative contracts	$	111.3	$	—	$	111.3	$	—
Contingent consideration obligations	523.6		—		—		523.6
Total	$	634.9	$	—	$	111.3	$	523.6

Summary of fair and carrying value of debt instruments

The fair and carrying values of our debt instruments, which are Level 2 liabilities, are summarized as follows:



	As of September 30, 2018				As of December 31, 2017
(In millions)	Fair Value		Carrying Value		Fair Value		Carrying Value
Notes payable to Fumedica AG	$	—	$	—	$	3.2	$	3.2
2.900% Senior Notes due September 15, 2020	1,491.6		1,476.2		1,517.7		1,482.4
3.625% Senior Notes due September 15, 2022	1,001.7		995.2		1,032.9		994.3
4.050% Senior Notes due September 15, 2025	1,756.6		1,737.4		1,851.9		1,736.3
5.200% Senior Notes due September 15, 2045	1,861.1		1,722.3		2,077.6		1,722.0
Total	$	6,111.0	$	5,931.1	$	6,483.3	$	5,938.2

Fair value of contingent consideration obligations

The following table provides a roll forward of the fair values of our contingent consideration obligations, which includes Level 3 measurements:



	For the Three Months Ended September 30,					For the Nine Months Ended September 30,
(In millions)	2018			2017		2018			2017
Fair value, beginning of period	$	499.9		$	492.1	$	523.6		$	467.6
Changes in fair value	(87.9		)	30.0		(91.6		)	61.2
Payments	—			—		(20.0		)	(6.7		)
Fair value, end of period	$	412.0		$	522.1	$	412.0		$	522.1

Financial Instruments (Tables)

9 Months Ended

Sep. 30, 2018

Investments, Debt and Equity Securities [Abstract]

Summary of financial assets with maturities of less than 90 days included within cash and cash equivalents

The following table summarizes our financial assets with maturities of less than 90 days from the date of purchase included in cash and cash equivalents in our condensed consolidated balance sheets:



(In millions)	As of September 30, 2018		As of December 31, 2017
Commercial paper	$	125.3	$	30.5
Overnight reverse repurchase agreements	23.7		3.6
Money market funds	1,695.1		948.0
Short-term debt securities	56.8		247.3
Total	$	1,900.9	$	1,229.4

Marketable debt and equity securities

The following tables summarize our marketable debt and equity securities, classified as available-for-sale:



As of September 30, 2018 (In millions)	Fair Value		Gross Unrealized Gains		Gross Unrealized Losses			Amortized Cost
Corporate debt securities
Current	$	1,273.7	$	0.1	$	(0.4	)	$	1,274.0
Non-current	677.9		0.5		(0.8		)	678.2
Government securities
Current	767.9		—		(0.5		)	768.4
Non-current	293.0		—		(0.6		)	293.6
Mortgage and other asset backed securities
Current	0.1		—		—			0.1
Non-current	273.6		0.1		(0.6		)	274.1
Total marketable debt securities	$	3,286.2	$	0.7	$	(2.9	)	$	3,288.4
Marketable equity securities, non-current	$	624.8	$	132.6	$	(4.4	)	$	496.6



As of December 31, 2017 (In millions)	Fair Value		Gross Unrealized Gains		Gross Unrealized Losses			Amortized Cost
Corporate debt securities
Current	$	1,039.3	$	—	$	(0.2	)	$	1,039.5
Non-current	1,570.5		0.9		—			1,569.6
Government securities
Current	1,075.1		0.1		(0.7		)	1,075.7
Non-current	844.2		0.2		(1.1		)	845.1
Mortgage and other asset backed securities
Current	0.8		—		—			0.8
Non-current	642.6		1.1		(0.8		)	642.3
Total marketable debt securities	$	5,172.5	$	2.3	$	(2.8	)	$	5,173.0
Marketable equity securities, non-current	$	11.8	$	1.8	$	(4.4	)	$	14.4

Summary of contractual maturities: available-for-sale securities

The estimated fair value and amortized cost of our marketable debt securities available-for-sale by contractual maturity are summarized as follows:



	As of September 30, 2018				As of December 31, 2017
(In millions)	Estimated Fair Value		Amortized Cost		Estimated Fair Value		Amortized Cost
Due in one year or less	$	2,041.7	$	2,042.5	$	2,115.2	$	2,116.0
Due after one year through five years	1,111.2		1,112.3		2,730.0		2,730.0
Due after five years	133.3		133.6		327.3		327.0
Total available-for-sale securities	$	3,286.2	$	3,288.4	$	5,172.5	$	5,173.0

Proceeds from marketable debt securities

The proceeds from maturities and sales of marketable debt securities and resulting realized gains and losses are summarized as follows:



	For the Three Months Ended September 30,						For the Nine Months Ended September 30,
(In millions)	2018			2017			2018			2017
Proceeds from maturities and sales	$	1,192.0		$	888.1		$	7,994.7		$	4,472.6
Realized gains	$	0.4		$	0.3		$	3.0		$	2.7
Realized losses	$	(0.6	)	$	(1.2	)	$	(10.8	)	$	(4.4	)

Derivative Instruments (Tables)

9 Months Ended

Sep. 30, 2018

Derivative Instruments and Hedging Activities Disclosure [Abstract]

Foreign currency forward contracts that were entered into to hedge forecasted revenue

The notional value of foreign currency forward contracts that were entered into to hedge forecasted revenues and operating expenses is summarized as follows:



	Notional Amount
Foreign Currency: (In millions)	As of September 30, 2018		As of December 31, 2017
Euro	$	1,662.5	$	1,875.6
British pound	34.9		150.9
Canadian dollar	26.9		83.5
Swiss franc	9.4		88.7
Total foreign currency forward contracts	$	1,733.7	$	2,198.7

Summary of the effect of cash flow derivatives designated as hedging instruments on the condensed consolidated statements of income

The following tables summarize the effect of foreign currency forward contracts designated as hedging instruments in our condensed consolidated statements of income:



For the Three Months Ended September 30, 2018							For the Nine Months Ended September 30, 2018
Net Gains/(Losses) Reclassified from AOCI into Operating Income (in millions)				Net Gains/(Losses) Recognized in Operating Income (in millions)			Net Gains/(Losses) Reclassified from AOCI into Operating Income (in millions)				Net Gains/(Losses) Recognized in Operating Income (in millions)
Location	2018			Location	2018		Location	2018			Location	2018
Revenues	$	(8.4	)	Revenues	$	0.3	Revenues	$	(51.7	)	Revenues	$	7.3
Operating expenses	$	(0.3	)	Operating expenses	$	0.4	Operating expenses	$	0.6		Operating expenses	$	—



For the Three Months Ended September 30, 2017							For the Nine Months Ended September 30, 2017
Net Gains/(Losses) Reclassified from AOCI into Operating Income (in millions)				Net Gains/(Losses) Recognized Directly into Net Income (in millions)			Net Gains/(Losses) Reclassified from AOCI into Operating Income (in millions)				Net Gains/(Losses) Recognized Directly into Net Income (in millions)
Location	2017			Location	2017		Location	2017			Location	2017
Revenues	$	(18.8	)	Other income (expense)	$	0.7	Revenue	$	(15.1	)	Other income (expense)	$	6.7
Operating expenses	$	0.5		Other income (expense)	$	0.2	Operating expenses	$	0.7		Other income (expense)	$	(0.1	)

Summary of fair value and presentation of derivatives



		Fair Value
(In millions)	Balance Sheet Location	As of September 30, 2018
Hedging Instruments:
Asset derivatives	Other current assets	$	25.6
	Investments and other assets	$	5.4
Liability derivatives	Accrued expenses and other	$	10.9
	Other long-term liabilities	$	18.7
Other Derivatives:
Asset derivatives	Other current assets	$	4.1
Liability derivatives	Accrued expenses and other	$	1.6



		Fair Value
(In millions)	Balance Sheet Location	As of December 31, 2017
Hedging Instruments:
Asset derivatives	Other current assets	$	0.7
	Investments and other assets	$	0.2
Liability derivatives	Accrued expenses and other	$	84.7
	Other long-term liabilities	$	23.6
Other Derivatives:
Asset derivatives	Other current assets	$	1.8
Liability derivatives	Accrued expenses and other	$	3.0

Equity (Tables)

9 Months Ended

Sep. 30, 2018

Equity [Abstract]

Reconciliation of equity attributable to noncontrolling interests

The following table reconciles equity (deficit) attributable to noncontrolling interests (NCI):



	For the Three Months Ended September 30,						For the Nine Months Ended September 30,
(In millions)	2018			2017			2018			2017
NCI, beginning of period	$	(7.2	)	$	(11.6	)	$	(14.7	)	$	(11.5	)
Net income (loss) attributable to NCI, net of tax	(1.5		)	—			45.2			(0.1		)
Capital contribution by noncontrolling interest	2.0			—			13.1			—
Distribution to noncontrolling interest	—			—			(50.0		)	—
Translation adjustment and other	(0.1		)	—			(0.4		)	—
NCI, end of period	$	(6.8	)	$	(11.6	)	$	(6.8	)	$	(11.6	)

Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) (Tables)

9 Months Ended

Sep. 30, 2018

Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]

Schedule of accumulated other comprehensive income (loss)

The following table summarizes the changes in accumulated other comprehensive income (loss), net of tax by component:



(In millions)	Unrealized Gains (Losses) on Securities Available for Sale, Net of Tax			Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax			Unfunded Status of Postretirement Benefit Plans, Net of Tax			Translation Adjustments			Total
Balance, December 31, 2017	$	(1.6	)	$	(104.5	)	$	(36.8	)	$	(175.5	)	$	(318.4	)
Amount reclassified, net of tax, upon adoption of ASU 2016-01	1.5			—			—			—			1.5
Balance, January 1, 2018	(0.1		)	(104.5		)	(36.8		)	(175.5		)	(316.9		)
Other comprehensive income (loss) before reclassifications	(7.9		)	58.3			0.2			(38.8		)	11.8
Amounts reclassified from accumulated other comprehensive income (loss)	6.1			50.7			—			—			56.8
Net current period other comprehensive income (loss)	(1.8		)	109.0			0.2			(38.8		)	68.6
Balance, September 30, 2018	$	(1.9	)	$	4.5		$	(36.6	)	$	(214.3	)	$	(248.3	)



(In millions)	Unrealized Gains (Losses) on Securities Available for Sale, Net of Tax			Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax			Unfunded Status of Postretirement Benefit Plans, Net of Tax			Translation Adjustments			Total
Balance, December 31, 2016	$	(10.8	)	$	57.8		$	(32.7	)	$	(334.2	)	$	(319.9	)
Other comprehensive income (loss) before reclassifications	5.5			(176.5		)	(0.5		)	146.7			(24.8		)
Amounts reclassified from accumulated other comprehensive income (loss)	1.1			14.2			—			—			15.3
Net current period other comprehensive income (loss)	6.6			(162.3		)	(0.5		)	146.7			(9.5		)
Balance, September 30, 2017	$	(4.2	)	$	(104.5	)	$	(33.2	)	$	(187.5	)	$	(329.4	)

Reclassification out of accumulated other comprehensive income

The following table summarizes the amounts reclassified from accumulated other comprehensive income:



(In millions)	Income Statement Location	Amounts Reclassified from Accumulated Other Comprehensive Income
		For the Three Months Ended September 30,						For the Nine Months Ended September 30,
		2018			2017			2018			2017
Gains (losses) on securities available for sale	Other income (expense)	$	(0.1	)	$	(0.9	)	$	(7.7	)	$	(1.7	)
	Income tax benefit (expense)	—			0.3			1.6			0.6

Gains (losses) on cash flow hedges	Revenues	(8.4		)	(18.8		)	(51.7		)	(15.1		)
	Operating expenses	(0.3		)	0.5			0.6			0.7
	Other income (expense)	0.1			0.1			0.2			0.2
	Income tax benefit (expense)	—			—			0.2			—

Total reclassifications, net of tax		$	(8.7	)	$	(18.8	)	$	(56.8	)	$	(15.3	)

Earnings per Share (Tables)

9 Months Ended

Sep. 30, 2018

Earnings Per Share [Abstract]

Basic and diluted earnings per share

Basic and diluted earnings per share are calculated as follows:



	For the Three Months Ended September 30,				For the Nine Months Ended September 30,
(In millions)	2018		2017		2018		2017
Numerator:
Net income attributable to Biogen Inc.	$	1,444.4	$	1,226.1	$	3,483.9	$	2,836.5
Denominator:
Weighted-average number of common shares outstanding	201.4		211.4		206.6		213.0
Effect of dilutive securities:
Stock options and employee stock purchase plan	—		0.1		—		—
Time-vested restricted stock units	0.4		0.2		0.3		0.2
Market stock units	0.1		0.1		0.1		0.1
Performance stock units settled in stock	—		—		—		—
Dilutive potential common shares	0.5		0.4		0.4		0.3
Shares used in calculating diluted earnings per share	201.9		211.8		207.0		213.3

Share-Based Payments (Tables)

9 Months Ended

Sep. 30, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Share-based compensation expense included in condensed consolidated statements of income

The following table summarizes share-based compensation expense included in our condensed consolidated statements of income:



	For the Three Months Ended September 30,						For the Nine Months Ended September 30,
(In millions)	2018			2017			2018			2017
Research and development	$	21.0		$	19.5		$	60.5		$	55.7
Selling, general and administrative	29.4			23.6			83.1			71.9
Subtotal	50.4			43.1			143.6			127.6
Capitalized share-based compensation costs	(3.5		)	(2.5		)	(9.7		)	(7.6		)
Share-based compensation expense included in total cost and expenses	46.9			40.6			133.9			120.0
Income tax effect	(7.7		)	(10.9		)	(21.8		)	(31.8		)
Share-based compensation expense included in net income attributable to Biogen Inc.	$	39.2		$	29.7		$	112.1		$	88.2

Summary of share-based compensation expense associated with each of our share-based compensating programs

The following table summarizes share-based compensation expense associated with each of our share-based compensation programs:



	For the Three Months Ended September 30,						For the Nine Months Ended September 30,
(In millions)	2018			2017			2018			2017
Market stock units	$	7.4		$	4.5		$	20.6		$	17.1
Time-vested restricted stock units	29.9			26.7			96.6			81.2
Cash settled performance units	5.8			7.0			9.4			13.0
Performance units	3.0			3.2			4.2			8.9
Performance stock units settled in stock	1.3			—			3.4			—
Performance stock units settled in cash	1.1			—			1.5			—
Employee stock purchase plan	1.9			1.7			7.9			7.4
Subtotal	50.4			43.1			143.6			127.6
Capitalized share-based compensation costs	(3.5		)	(2.5		)	(9.7		)	(7.6		)
Share-based compensation expense included in total cost and expenses	$	46.9		$	40.6		$	133.9		$	120.0

Income Taxes (Tables)

9 Months Ended

Sep. 30, 2018

Income Tax Disclosure [Abstract]

Reconciliation between the U.S. federal statutory tax rate and effective tax rate

A reconciliation between the U.S. federal statutory tax rate and our effective tax rate is summarized as follows:



	For the Three Months Ended September 30,				For the Nine Months Ended September 30,
	2018		2017		2018		2017
Statutory rate	21.0	%	35.0	%	21.0	%	35.0	%
State taxes	0.6		0.7		0.7		0.6
Taxes on foreign earnings	0.8		(11.4	)	0.1		(11.4	)
Credits and net operating loss utilization	(1.0	)	(0.7	)	(0.8	)	(0.8	)
Purchased intangible assets	0.3		1.2		0.5		1.3
Manufacturing deduction	—		(2.0	)	—		(2.1	)
Other permanent items	0.3		0.6		0.3		0.7
Tax reform	(0.6	)	—		(0.3	)	—
Other	(1.0	)	0.4		(0.2	)	0.6
Effective tax rate	20.4	%	23.8	%	21.3	%	23.9	%

Other Consolidated Financial Statement Detail (Tables)

9 Months Ended

Sep. 30, 2018

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Other income (expense), net

Components of other income (expense), net, are summarized as follows:



	For the Three Months Ended September 30,						For the Nine Months Ended September 30,
(In millions)	2018			2017			2018			2017
Interest income	$	26.0		$	20.6		$	81.4		$	54.2
Interest expense	(49.0		)	(61.8		)	(151.2		)	(188.8		)
Gain (loss) on investments, net	141.1			(4.0		)	132.0			(15.0		)
Foreign exchange gains (losses), net	0.2			6.7			(13.8		)	8.4
Other, net	(3.2		)	(5.5		)	(8.8		)	(9.4		)
Total other income (expense), net	$	115.1		$	(44.0	)	$	39.6		$	(150.6	)

Accrued expenses and other

Accrued expenses and other consists of the following:



(In millions)	As of September 30, 2018		As of December 31, 2017
Revenue-related reserves for discounts and allowances	$	758.5	$	572.0
Current portion of contingent consideration obligations	381.5		844.6
Employee compensation and benefits	274.1		297.7
Royalties and licensing fees	226.3		206.7
Construction in progress	196.6		159.7
Collaboration expenses	101.5		183.7
Other	640.0		636.9
Total accrued expenses and other	$	2,578.5	$	2,901.3

Collaborative and Other Relationships Collaborative and Other Relationships (Tables)

9 Months Ended

Sep. 30, 2018

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Revenues from anti-CD20 therapeutic programs

Revenues from anti-CD20 therapeutic programs are summarized as follows:



	For the Three Months Ended September 30,				For the Nine Months Ended September 30,
(In millions)	2018		2017		2018		2017
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA	$	358.0	$	325.1	$	1,066.6	$	996.1
Other revenues from anti-CD20 therapeutic programs	153.7		81.4		378.7		148.1
Total revenues from anti-CD20 therapeutic programs	$	511.7	$	406.5	$	1,445.3	$	1,144.2

Summary of Activity Related to BAN2401 and Elenbecestat Collaboration

A summary of development expenses related to the BAN2401 and Elenbecestat Collaboration is as follows:



	For the Three Months Ended September 30,				For the Nine Months Ended September 30,
(In millions)	2018		2017		2018		2017
Total development expense incurred by the collaboration related to the advancement of BAN2401 and Elenbecestat	$	64.9	$	38.3	$	176.0	$	105.0
Biogen's share of BAN2401 and Elenbecestat development expense reflected in research and development expense in our condensed consolidated statements of income	$	32.5	$	19.2	$	88.0	$	52.5

Summary of Activity Related to Aducanumab Collaboration

A summary of development and sales and marketing expenses related to the Aducanumab Collaboration Agreement is as follows:



	For the Three Months Ended September 30,				For the Nine Months Ended September 30,
(In millions)	2018		2017		2018		2017
Total aducanumab development expense	$	64.8	$	73.3	$	204.8	$	202.2
Biogen's share of aducanumab development expense reflected in research and development expense in our condensed consolidated statements of income	$	55.1	$	73.3	$	183.6	$	202.2

Total aducanumab sales and marketing expense incurred by the collaboration	$	12.1	$	5.1	$	33.3	$	14.9
Biogen's share of aducanumab sales and marketing expense reflected in selling, general and administrative expense our condensed consolidated statements of income	$	5.1	$	5.1	$	19.0	$	14.9

Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Details) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2018 segment	Sep. 30, 2017 USD ($)	Jan. 01, 2018 USD ($)
Number of reportable segments \| segment		1
Interest in subsidiary (less than given percentage)		100.00%
Accounting Standards Update 2016-01
Cumulative effect of new accounting principle				$ 1.5
Accounting Standards Update 2016-01 \| Retained earnings
Cumulative effect of new accounting principle				0.0
Accounting Standards Update 2016-16 \| Retained earnings
Cumulative effect of new accounting principle				500.0
Accounting Standards Update 2016-16 \| Net deferred tax asset
Cumulative effect of new accounting principle				$ 500.0
Accounting Standards Update 2017-08 \| Other income (expense)
Prior period reclassification adjustment	$ 0.4		$ 1.2

Acquisitions (Details Textual) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
Acquisitions (Details Textual) - USD ($) $ in Millions	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Business Acquisition [Line Items]
Acquired in-process research and development	$ 27.5			$ 0.0	$ 112.5	$ 120.0
Karyopharm
Business Acquisition [Line Items]
Acquired in-process research and development			$ 10.0
Estimated additional payments upon achievement of milestones					207.0
Pfizer
Business Acquisition [Line Items]
Acquired in-process research and development		$ 75.0
Estimated additional payments upon achievement of milestones					515.0
TMS
Business Acquisition [Line Items]
Acquired in-process research and development					18.0
Estimated additional payments upon achievement of milestones					335.0
Research and development expense asset acquired		$ 4.0
AliveGen
Business Acquisition [Line Items]
Acquired in-process research and development	$ 27.5
Estimated additional payments upon achievement of milestones					535.0
Development Milestones \| Pfizer
Business Acquisition [Line Items]
Estimated additional payments upon achievement of milestones					$ 10.0

Restructuring Restructuring (Details Textual) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 6.0		$ 0.0	$ 9.2	$ 0.0
2017 Corporate Strategy
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 6.0	$ 0.9		$ 9.2

Revenues Revenues by Product (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Product revenues	$ 3,439.0	$ 3,077.8	$ 9,926.6	$ 8,966.9
TECFIDERA
Disaggregation of Revenue [Line Items]
Product revenues	1,090.0	1,069.6	3,163.7	3,138.4
TECFIDERA \| U.S.
Disaggregation of Revenue [Line Items]
Product revenues	842.1	836.3	2,396.8	2,462.4
TECFIDERA \| Rest of world
Disaggregation of Revenue [Line Items]
Product revenues	247.9	233.3	766.9	676.0
Interferon
Disaggregation of Revenue [Line Items]
Product revenues	590.1	662.0	1,765.9	2,000.9
Interferon \| U.S.
Disaggregation of Revenue [Line Items]
Product revenues	421.5	473.3	1,237.5	1,439.8
Interferon \| Rest of world
Disaggregation of Revenue [Line Items]
Product revenues	168.6	188.7	528.4	561.1
TYSABRI
Disaggregation of Revenue [Line Items]
Product revenues	470.2	469.4	1,399.5	1,510.4
TYSABRI \| U.S.
Disaggregation of Revenue [Line Items]
Product revenues	253.0	266.8	768.2	861.7
TYSABRI \| Rest of world
Disaggregation of Revenue [Line Items]
Product revenues	217.2	202.6	631.3	648.7
FAMPYRA
Disaggregation of Revenue [Line Items]
Product revenues	22.5	24.3	69.9	67.4
FAMPYRA \| U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
FAMPYRA \| Rest of world
Disaggregation of Revenue [Line Items]
Product revenues	22.5	24.3	69.9	67.4
ZINBRYTA
Disaggregation of Revenue [Line Items]
Product revenues	0.0	14.2	1.4	41.0
ZINBRYTA \| U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
ZINBRYTA \| Rest of world
Disaggregation of Revenue [Line Items]
Product revenues	0.0	14.2	1.4	41.0
SPINRAZA
Disaggregation of Revenue [Line Items]
Product revenues	467.7	270.9	1,254.3	521.2
SPINRAZA \| U.S.
Disaggregation of Revenue [Line Items]
Product revenues	223.9	197.6	617.8	438.8
SPINRAZA \| Rest of world
Disaggregation of Revenue [Line Items]
Product revenues	243.8	73.3	636.5	82.4
ELOCTATE
Disaggregation of Revenue [Line Items]
Product revenues			0.0	48.4
ELOCTATE \| U.S.
Disaggregation of Revenue [Line Items]
Product revenues			0.0	42.2
ELOCTATE \| Rest of world
Disaggregation of Revenue [Line Items]
Product revenues			0.0	6.2
ALPROLIX
Disaggregation of Revenue [Line Items]
Product revenues			0.0	26.0
ALPROLIX \| U.S.
Disaggregation of Revenue [Line Items]
Product revenues			0.0	21.0
ALPROLIX \| Rest of world
Disaggregation of Revenue [Line Items]
Product revenues			0.0	5.0
FUMADERM
Disaggregation of Revenue [Line Items]
Product revenues	4.8	10.7	17.3	30.7
FUMADERM \| U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
FUMADERM \| Rest of world
Disaggregation of Revenue [Line Items]
Product revenues	4.8	10.7	17.3	30.7
BENEPALI
Disaggregation of Revenue [Line Items]
Product revenues	123.4	99.2	359.9	253.2
BENEPALI \| U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
BENEPALI \| Rest of world
Disaggregation of Revenue [Line Items]
Product revenues	123.4	99.2	359.9	253.2
FLIXABI
Disaggregation of Revenue [Line Items]
Product revenues	11.4	2.2	29.2	4.7
FLIXABI \| U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0.0	0.0	0.0	0.0
FLIXABI \| Rest of world
Disaggregation of Revenue [Line Items]
Product revenues	11.4	2.2	29.2	4.7
Product, net
Disaggregation of Revenue [Line Items]
Product revenues	2,780.1	2,622.5	8,061.1	7,642.3
Product, net \| U.S.
Disaggregation of Revenue [Line Items]
Product revenues	1,740.5	1,774.0	5,020.3	5,265.9
Product, net \| Rest of world
Disaggregation of Revenue [Line Items]
Product revenues	$ 1,039.6	$ 848.5	$ 3,040.8	$ 2,376.4

Revenues Reserves for Discounts and Allowances (Details 1) $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($)
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	$ 761.6
Current Provisions Relating To Sales In Current Year	2,466.5
Adjustments Relating To Prior Years	(7.2)
Payments/Returns Relating To Sales in Current Year	(1,660.3)
Payments/Returns Relating To Sales in Prior Year	(624.8)
Ending Balance	935.8
Discounts
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	109.6
Current Provisions Relating To Sales In Current Year	498.9
Adjustments Relating To Prior Years	(0.3)
Payments/Returns Relating To Sales in Current Year	(373.7)
Payments/Returns Relating To Sales in Prior Year	(107.8)
Ending Balance	126.7
Contractual adjustments
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	606.0
Current Provisions Relating To Sales In Current Year	1,949.6
Adjustments Relating To Prior Years	(7.0)
Payments/Returns Relating To Sales in Current Year	(1,285.8)
Payments/Returns Relating To Sales in Prior Year	(495.2)
Ending Balance	767.6
Returns
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	46.0
Current Provisions Relating To Sales In Current Year	18.0
Adjustments Relating To Prior Years	0.1
Payments/Returns Relating To Sales in Current Year	(0.8)
Payments/Returns Relating To Sales in Prior Year	(21.8)
Ending Balance	$ 41.5

Revenues Reserves for Discounts and Allowances (Details 2) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Total Reserves	$ 935.8	$ 761.6
Reduction of accounts receivable
Total Reserves	177.3	189.6
Component of accrued expenses and other
Total Reserves	$ 758.5	$ 572.0

Revenues Revenues from Anti-CD20 Therapeutic Programs (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	$ 3,439.0	$ 3,077.8	$ 9,926.6	$ 8,966.9
Genentech
Disaggregation of Revenue [Line Items]
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA	358.0	325.1	1,066.6	996.1
Other revenues from anti-CD20 therapeutic programs	153.7	81.4	378.7	148.1
Revenues from anti-CD20 therapeutic programs
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	$ 511.7	$ 406.5	$ 1,445.3	$ 1,144.2

Revenues Other Revenues (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
Revenues Other Revenues (Details) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Total revenues	$ 3,439.0	$ 3,077.8	$ 9,926.6	$ 8,966.9
Royalty
Disaggregation of Revenue [Line Items]
Total revenues	7.9	11.8	35.8	49.1
Other corporate revenues
Disaggregation of Revenue [Line Items]
Total revenues	91.9	30.8	311.6	109.8
Other
Disaggregation of Revenue [Line Items]
Total revenues	147.2	48.8	420.2	180.4
Collaborative arrangement \| AbbVie
Disaggregation of Revenue [Line Items]
Total revenues	(0.7)	(2.8)	(7.9)	(12.6)
Collaborative arrangement \| Samsung Bioepis
Disaggregation of Revenue [Line Items]
Total revenues	$ 48.1	$ 9.0	$ 80.7	$ 34.1

Revenues Revenues (Details Textual)	3 Months Ended		9 Months Ended
Revenues Revenues (Details Textual)	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Distributor One
Disaggregation of Revenue [Line Items]
Percentage of revenues from major distributors	30.70%	33.70%	32.40%	34.90%
Distributor Two
Disaggregation of Revenue [Line Items]
Percentage of revenues from major distributors	19.30%	20.80%	18.00%	21.00%

Inventory (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Components of inventories
Raw materials	$ 181.8	$ 162.4
Work in process	608.0	605.7
Finished goods	136.5	157.4
Total inventory	926.3	925.5
Inventory, current	916.6	902.7
Inventory, Noncurrent	$ 9.7	$ 22.8

Intangible Assets and Goodwill (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Intangible assets
Total intangible assets, gross	$ 8,067.7			$ 8,067.7		$ 8,264.6
Accumulated Amortization	(4,688.7)			(4,688.7)		(4,385.0)
Intangible assets, net	3,379.0			3,379.0		3,879.6
Contingent consideration	87.9	$ (30.0)		91.6	$ (61.2)
Amortization of acquired intangible assets	281.9	$ 108.9		493.2	$ 674.9
Payment made to Forward Pharma			$ 1,250.0
Intangible Assets and Goodwill (Textual) [Abstract]
Accumulated impairment losses related to goodwill	0.0			0.0
Expected future amortization expense, 2018 (remaining three months)	91.7			91.7
Expected future amortization expense, 2019	367.0			367.0
Expected future amortization expense, 2020	366.8			366.8
Expected future amortization expense, 2021	249.4			249.4
Expected future amortization expense, 2022	250.5			250.5
Expected future amortization expense, 2023	230.0			230.0
Out-licensed patents
Intangible assets
Cost	543.3			543.3		543.3
Net	1.7			1.7		7.7
Accumulated Amortization	(541.6)			$ (541.6)		(535.6)
Out-licensed patents \| Minimum
Intangible assets
Estimated life, (in years)				13 years
Out-licensed patents \| Maximum
Intangible assets
Estimated life, (in years)				23 years
Developed technology
Intangible assets
Cost	3,005.3			$ 3,005.3		3,005.3
Net	281.3			281.3		316.3
Accumulated Amortization	(2,724.0)			$ (2,724.0)		(2,689.0)
Developed technology \| Minimum
Intangible assets
Estimated life, (in years)				15 years
Developed technology \| Maximum
Intangible assets
Estimated life, (in years)				23 years
Developed technology \| AVONEX
Intangible assets
Net	275.3			$ 275.3
Acquired and in-licensed rights and patents
Intangible assets
Cost	3,974.2			3,974.2		3,971.4
Net	2,551.1			2,551.1		2,811.0
Accumulated Amortization	(1,423.1)			$ (1,423.1)		(1,160.4)
Acquired and in-licensed rights and patents \| Minimum
Intangible assets
Estimated life, (in years)				4 years
Acquired and in-licensed rights and patents \| Maximum
Intangible assets
Estimated life, (in years)				18 years
Acquired and in-licensed rights and patents \| TYSABRI
Intangible assets
Net	2,073.6			$ 2,073.6
Acquired and in-licensed rights and patents \| TECFIDERA
Intangible assets
Net	262.6		795.2	$ 262.6
Amortization of acquired intangible assets			$ 328.2
In Process Research and Development
Intangible assets
Indefinite lived intangible assets useful life				Indefinite until commercialization
Cost and Net	480.9			$ 480.9		680.6
Accumulated Amortization	0.0			$ 0.0		0.0
Amortization of acquired intangible assets	189.3
Trademarks and Trade Names
Intangible assets
Indefinite lived intangible assets useful life				Indefinite
Cost and Net	64.0			$ 64.0		64.0
Accumulated Amortization	0.0			0.0		$ 0.0
PLSR \| In Process Research and Development
Intangible assets
Amortization of acquired intangible assets	60.0
TGN
Intangible assets
Contingent consideration	89.6
TGN \| In Process Research and Development
Intangible assets
Cost and Net	41.8			$ 41.8
Amortization of acquired intangible assets	$ 129.3

Intangible Assets and Goodwill (Details 1) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Goodwill [Line Items]
Increase in goodwill			$ 810.7
Income tax expense (benefit)	$ 369.8	$ 383.8	956.0	$ 892.6
Summary of roll forward of the changes in goodwill
Goodwill, beginning of period			4,632.5
Other			(3.1)
Goodwill, end of period	$ 5,440.1		5,440.1
TECFIDERA \| Fumapharm AG
Goodwill [Line Items]
Increase in goodwill			900.0
Income tax expense (benefit)			$ 89.3

Fair Value Measurements (Details) - USD ($) $ in Millions	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Assets:
Marketable debt securities	$ 3,286.2		$ 5,172.5
Fair Value, Measurements, Recurring
Assets:
Cash equivalents	1,900.9		1,229.4
Marketable equity securities	624.8		11.8
Derivative contracts	35.1		2.7
Plan assets for deferred compensation	26.4		28.5
Total	5,873.4		6,444.9
Liabilities:
Derivative contracts	31.2		111.3
Contingent consideration obligations	412.0	$ 499.9	523.6	$ 522.1	$ 492.1	$ 467.6
Total	443.2		634.9
Fair Value, Measurements, Recurring \| Corporate debt securities
Assets:
Marketable debt securities	1,951.6		2,609.8
Fair Value, Measurements, Recurring \| Government securities
Assets:
Marketable debt securities	1,060.9		1,919.3
Fair Value, Measurements, Recurring \| Mortgage and other asset backed securities
Assets:
Marketable debt securities	273.7		643.4
Quoted Prices in Active Markets (Level 1) \| Fair Value, Measurements, Recurring
Assets:
Cash equivalents	0.0		0.0
Marketable equity securities	95.6		11.8
Derivative contracts	0.0		0.0
Plan assets for deferred compensation	0.0		0.0
Total	95.6		11.8
Liabilities:
Derivative contracts	0.0		0.0
Contingent consideration obligations	0.0		0.0
Total	0.0		0.0
Quoted Prices in Active Markets (Level 1) \| Fair Value, Measurements, Recurring \| Corporate debt securities
Assets:
Marketable debt securities	0.0		0.0
Quoted Prices in Active Markets (Level 1) \| Fair Value, Measurements, Recurring \| Government securities
Assets:
Marketable debt securities	0.0		0.0
Quoted Prices in Active Markets (Level 1) \| Fair Value, Measurements, Recurring \| Mortgage and other asset backed securities
Assets:
Marketable debt securities	0.0		0.0
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring
Assets:
Cash equivalents	1,900.9		1,229.4
Marketable equity securities	529.2		0.0
Derivative contracts	35.1		2.7
Plan assets for deferred compensation	26.4		28.5
Total	5,777.8		6,433.1
Liabilities:
Derivative contracts	31.2		111.3
Contingent consideration obligations	0.0		0.0
Total	31.2		111.3
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Corporate debt securities
Assets:
Marketable debt securities	1,951.6		2,609.8
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Government securities
Assets:
Marketable debt securities	1,060.9		1,919.3
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Mortgage and other asset backed securities
Assets:
Marketable debt securities	273.7		643.4
Significant Unobservable Inputs (Level 3) \| Fair Value, Measurements, Recurring
Assets:
Cash equivalents	0.0		0.0
Marketable equity securities	0.0		0.0
Derivative contracts	0.0		0.0
Plan assets for deferred compensation	0.0		0.0
Total	0.0		0.0
Liabilities:
Derivative contracts	0.0		0.0
Contingent consideration obligations	412.0		523.6
Total	412.0		523.6
Significant Unobservable Inputs (Level 3) \| Fair Value, Measurements, Recurring \| Corporate debt securities
Assets:
Marketable debt securities	0.0		0.0
Significant Unobservable Inputs (Level 3) \| Fair Value, Measurements, Recurring \| Government securities
Assets:
Marketable debt securities	0.0		0.0
Significant Unobservable Inputs (Level 3) \| Fair Value, Measurements, Recurring \| Mortgage and other asset backed securities
Assets:
Marketable debt securities	$ 0.0		$ 0.0

Fair Value Measurements (Details 1) SFr in Millions, $ in Millions	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2006 CHF (SFr)
Debt Instrument [Line Items]
Notes payable, carrying value	$ 5,931.1	$ 5,938.2
Debt instruments, fair value	6,111.0	6,483.3
Notes Payable to Fumedica
Debt Instrument [Line Items]
Notes payable, fair value	0.0	3.2
Notes payable, carrying value	0.0	3.2	SFr 61.4
2.900% Senior Notes due 2020
Debt Instrument [Line Items]
Notes payable, fair value	1,491.6	1,517.7
Notes payable, carrying value	1,476.2	1,482.4
3.625% Senior Notes due 2022
Debt Instrument [Line Items]
Notes payable, fair value	1,001.7	1,032.9
Notes payable, carrying value	995.2	994.3
4.050% Senior Notes due 2025
Debt Instrument [Line Items]
Notes payable, fair value	1,756.6	1,851.9
Notes payable, carrying value	1,737.4	1,736.3
5.200% Senior Notes due 2045
Debt Instrument [Line Items]
Notes payable, fair value	1,861.1	2,077.6
Notes payable, carrying value	$ 1,722.3	$ 1,722.0

Fair Value Measurements (Details 2) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
Fair Value Measurements (Details 2) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration	$ (87.9)	$ 30.0	$ (91.6)	$ 61.2
Payments	0.0	0.0	(20.0)	(6.7)
Fair Value, Measurements, Recurring
Business Acquisition, Contingent Consideration [Line Items]
Fair value, beginning of period	499.9	492.1	523.6	467.6
Fair value, end of period	$ 412.0	$ 522.1	$ 412.0	$ 522.1

Fair Value Measurements (Details Textual) SFr in Millions, $ in Millions	1 Months Ended	3 Months Ended			9 Months Ended
	Oct. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2006 CHF (SFr)
Business Acquisition, Contingent Consideration [Line Items]
Asset impairment charges					$ 0.0
Fair value measurements, changes in valuation inputs					0
Notes payable		$ 5,931.1			$ 5,931.1		$ 5,938.2
Contingent consideration obligations		330.5			330.5		279.0
Future payment		0.0		$ 0.0	20.0	$ 6.7
Notes Payable To Fumedica
Business Acquisition, Contingent Consideration [Line Items]
Notes payable		$ 0.0			$ 0.0		3.2	SFr 61.4
2.900% Senior Notes due 2020
Business Acquisition, Contingent Consideration [Line Items]
Senior Notes interest rate		2.90%			2.90%
Notes payable		$ 1,476.2			$ 1,476.2		1,482.4
3.625% Senior Notes due 2022
Business Acquisition, Contingent Consideration [Line Items]
Senior Notes interest rate		3.625%			3.625%
Notes payable		$ 995.2			$ 995.2		994.3
4.050% Senior Notes due 2025
Business Acquisition, Contingent Consideration [Line Items]
Senior Notes interest rate		4.05%			4.05%
Notes payable		$ 1,737.4			$ 1,737.4		1,736.3
5.200% Senior Notes due 2045
Business Acquisition, Contingent Consideration [Line Items]
Senior Notes interest rate		5.20%			5.20%
Notes payable		$ 1,722.3			$ 1,722.3		$ 1,722.0
Ionis Pharmaceuticals
Business Acquisition, Contingent Consideration [Line Items]
Term of collaboration agreement			10 years
Expected contingent milestone payment
Business Acquisition, Contingent Consideration [Line Items]
Future payment	$ (81.5)

Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	$ 1,900.9	$ 1,229.4
Commercial paper
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	125.3	30.5
Overnight reverse repurchase agreements
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	23.7	3.6
Money market funds
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	1,695.1	948.0
Short-term debt securities
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	$ 56.8	$ 247.3

Financial Instruments (Details 1) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Fair value	$ 3,286.2	$ 5,172.5
Gross unrealized gains	0.7	2.3
Gross unrealized losses	(2.9)	(2.8)
Amortized cost	3,288.4	5,173.0
Corporate debt securities Current
Debt Securities, Available-for-sale [Line Items]
Fair value	1,273.7	1,039.3
Gross unrealized gains	0.1	0.0
Gross unrealized losses	(0.4)	(0.2)
Amortized cost	1,274.0	1,039.5
Corporate debt securities Non-current
Debt Securities, Available-for-sale [Line Items]
Fair value	677.9	1,570.5
Gross unrealized gains	0.5	0.9
Gross unrealized losses	(0.8)	0.0
Amortized cost	678.2	1,569.6
Government securities Current
Debt Securities, Available-for-sale [Line Items]
Fair value	767.9	1,075.1
Gross unrealized gains	0.0	0.1
Gross unrealized losses	(0.5)	(0.7)
Amortized cost	768.4	1,075.7
Government securities Non-current
Debt Securities, Available-for-sale [Line Items]
Fair value	293.0	844.2
Gross unrealized gains	0.0	0.2
Gross unrealized losses	(0.6)	(1.1)
Amortized cost	293.6	845.1
Mortgage and other asset backed securities Current
Debt Securities, Available-for-sale [Line Items]
Fair value	0.1	0.8
Gross unrealized gains	0.0	0.0
Gross unrealized losses	0.0	0.0
Amortized cost	0.1	0.8
Mortgage and other asset backed securities Non-current
Debt Securities, Available-for-sale [Line Items]
Fair value	273.6	642.6
Gross unrealized gains	0.1	1.1
Gross unrealized losses	(0.6)	(0.8)
Amortized cost	274.1	642.3
Marketable equity securities
Debt Securities, Available-for-sale [Line Items]
Marketable equity securities, fair value	624.8	11.8
Marketable equity securities, gross unrealized gains	132.6	1.8
Marketable equity securities, gross unrealized losses	(4.4)	(4.4)
Marketable equity securities, amortized cost	$ 496.6	$ 14.4

Financial Instruments (Details 2) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Summary of Contractual Maturities: Available-for-Sale Securities
Due in one year or less, estimated fair value	$ 2,041.7	$ 2,115.2
Due in one year or less, amortized cost	2,042.5	2,116.0
Due after one year through five years, estimated fair value	1,111.2	2,730.0
Due after one year through five years, amortized cost	1,112.3	2,730.0
Due after five years, estimated fair value	133.3	327.3
Due after five years, amortized cost	133.6	327.0
Fair value	3,286.2	5,172.5
Amortized cost	$ 3,288.4	$ 5,173.0

Financial Instruments (Details 3) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
Financial Instruments (Details 3) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Investments, Debt and Equity Securities [Abstract]
Proceeds from maturities and sales	$ 1,192.0	$ 888.1	$ 7,994.7	$ 4,472.6
Realized gains	0.4	0.3	3.0	2.7
Realized losses	$ (0.6)	$ (1.2)	$ (10.8)	$ (4.4)

Financial Instruments (Details Textual)	9 Months Ended	12 Months Ended
Financial Instruments (Details Textual)	Sep. 30, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Average maturity of marketable securities, months	11 months	17 months

Financial Instruments Financial Instruments (Details Textual 2) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Jun. 30, 2018	Sep. 30, 2018	Jun. 05, 2018	Dec. 31, 2017
Strategic Investments
Business Acquisition [Line Items]
Strategic investment portfolio		$ 687.8		$ 85.8
Ionis Pharmaceuticals
Business Acquisition [Line Items]
Term of collaboration agreement	10 years
Total payment to enter collaboration agreement			$ 1,000.0
Upfront payment for collaboration agreement			$ 375.0
Investment in common stock, shares purchased			11.5
Purchase of common stock			$ 625.0

Derivative Instruments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
Derivative Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Derivative Instruments (Textual) [Abstract]
Range of durations of foreign currency forward contracts			12 months
Gain/Loss on fair value of foreign currency forward contracts	$ 3.3		$ 3.3		$ (113.0)
Net gains (losses) of other income (expense) related to foreign currency forward contracts	(5.2)	$ 1.2	$ (4.8)	$ 5.7
Minimum
Derivative Instruments (Textual) [Abstract]
Range of durations of foreign currency forward contracts			1 month		1 month
Maximum
Derivative Instruments (Textual) [Abstract]
Range of durations of foreign currency forward contracts			15 months		21 months
Not designated as hedging instrument
Derivative Instruments (Textual) [Abstract]
Aggregate notional amount	691.7		$ 691.7		$ 564.9
Other current assets \| Designated as hedging instrument
Summary of Derivatives designated as Hedging Instruments
Derivative asset, fair value, net	25.6		25.6		0.7
Other current assets \| Not designated as hedging instrument
Summary of Derivatives designated as Hedging Instruments
Derivative asset, fair value, net	4.1		4.1		1.8
Investments and other assets \| Designated as hedging instrument
Summary of Derivatives designated as Hedging Instruments
Derivative asset, fair value, net	5.4		5.4		0.2
Accrued expenses and other \| Designated as hedging instrument
Summary of Derivatives designated as Hedging Instruments
Derivative liability, fair value, net	10.9		10.9		84.7
Accrued expenses and other \| Not designated as hedging instrument
Summary of Derivatives designated as Hedging Instruments
Derivative liability, fair value, net	1.6		1.6		3.0
Other long-term liabilities \| Designated as hedging instrument
Summary of Derivatives designated as Hedging Instruments
Derivative liability, fair value, net	18.7		18.7		23.6
Foreign exchange contract \| Designated as hedging instrument
Derivative Instruments (Textual) [Abstract]
Aggregate notional amount	1,733.7		1,733.7		2,198.7
Interest rate swap \| Designated as hedging instrument
Derivative Instruments (Textual) [Abstract]
Aggregate notional amount	675.0		675.0
Revenue \| Cash flows, revenue \| Foreign exchange contract
Derivative Instruments (Textual) [Abstract]
Net gains (losses) in operating income for the settlement of certain effective cash flow hedge instruments	(8.4)	(18.8)	(51.7)	(15.1)
Net gains (losses) in net income of foreign currency forward contracts due to exclusion from effectiveness testing	0.3		7.3
Operating expense \| Cash flows, operating expenses \| Foreign exchange contract
Derivative Instruments (Textual) [Abstract]
Net gains (losses) in operating income for the settlement of certain effective cash flow hedge instruments	(0.3)	0.5	0.6	0.7
Net gains (losses) in net income of foreign currency forward contracts due to exclusion from effectiveness testing	0.4		0.0
Other income (expense) \| Cash flows, revenue \| Foreign exchange contract
Derivative Instruments (Textual) [Abstract]
Net gains (losses) in net income of foreign currency forward contracts due to exclusion from effectiveness testing		0.7		6.7
Other income (expense) \| Cash flows, operating expenses \| Foreign exchange contract
Derivative Instruments (Textual) [Abstract]
Net gains (losses) in net income of foreign currency forward contracts due to exclusion from effectiveness testing		$ 0.2		$ (0.1)
Euro \| Foreign exchange contract \| Designated as hedging instrument
Derivative Instruments (Textual) [Abstract]
Aggregate notional amount	1,662.5		1,662.5		1,875.6
British pound \| Foreign exchange contract \| Designated as hedging instrument
Derivative Instruments (Textual) [Abstract]
Aggregate notional amount	34.9		34.9		150.9
Canadian dollar \| Foreign exchange contract \| Designated as hedging instrument
Derivative Instruments (Textual) [Abstract]
Aggregate notional amount	26.9		26.9		83.5
Swiss franc \| Foreign exchange contract \| Designated as hedging instrument
Derivative Instruments (Textual) [Abstract]
Aggregate notional amount	$ 9.4		$ 9.4		$ 88.7
2.900% Senior Notes due 2020
Derivative Instruments (Textual) [Abstract]
Senior Notes interest rate	2.90%		2.90%
Short-term derivative [Member]
Derivative Instruments (Textual) [Abstract]
Unrealized gain (loss) on foreign currency derivatives, net, before tax			$ 7.0

Property, Plant and Equipment Property, Plant and Equipment (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Accumulated depreciation	$ 1,730.6	$ 1,559.1
Solothurn, Switzerland
Property, Plant and Equipment [Line Items]
Construction in progress	1,600.0	$ 1,200.0
Contractual commitments for the construction of the facility	$ 200.0

Equity (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
Equity (Details) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reconciliation of equity attributable to non-controlling interests
NCI, beginning of period	$ (7.2)	$ (11.6)	$ (14.7)	$ (11.5)
Net income (loss) attributable to NCI, net of tax	(1.5)	0.0	45.2	(0.1)
Capital contribution by noncontrolling interest	2.0	0.0	13.1	0.0
Distribution to noncontrolling interest	0.0	0.0	(50.0)	0.0
Translation adjustment and other	(0.1)	0.0	(0.4)	0.0
NCI, end of period	$ (6.8)	$ (11.6)	$ (6.8)	$ (11.6)

Equity (Details Textual) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Sep. 30, 2016	Mar. 31, 2011
Class of Stock [Line Items]
Payments for repurchase of common stock			$ 3,000.0	$ 1,365.4
Equity increase during the period			1,200.0
Net income attributable to Biogen Inc.	$ 1,444.4	$ 1,226.1	3,483.9	$ 2,836.5
2018 Share Repurchase Program [Member]
Class of Stock [Line Items]
Authorized amount of share repurchases under the 2016 Share Repurchase Program	$ 3,500.0		$ 3,500.0
2016 Share Repurchase Program
Class of Stock [Line Items]
Authorized amount of share repurchases under the 2016 Share Repurchase Program						$ 5,000.0
Repurchase and retire of common stock, shares			10.5	3.7
Payments for repurchase of common stock			$ 3,000.0	$ 1,000.0
2011 Share Repurchase Program
Class of Stock [Line Items]
Common stock shares authorized for repurchased							20.0
Repurchase of common stock, shares				1.2
Payments for repurchase of common stock				$ 365.4
Accounting Standards Update 2016-16 \| Retained earnings
Class of Stock [Line Items]
Cumulative effect of new accounting principle					$ 500.0

Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018
Accumulated other comprehensive income [Line Items]
Balance, as of the beginning of the period			$ (318.4)	$ (319.9)
Balance, as of January 1, 2018	$ (248.3)	$ (329.4)	(318.4)	(319.9)	$ (316.9)
Other comprehensive income (loss) before reclassifications			11.8	(24.8)
Amounts reclassified from accumulated other comprehensive income (loss)			56.8	15.3
Net current period other comprehensive income (loss)	13.3	9.4	68.6	(9.5)
Balance, as of the end of the period	(248.3)	(329.4)	(248.3)	(329.4)
Accumulated net unrealized gains (losses) on securities available-for-sale
Accumulated other comprehensive income [Line Items]
Balance, as of the beginning of the period			(1.6)	(10.8)
Balance, as of January 1, 2018	(1.9)	(4.2)	(1.6)	(10.8)	(0.1)
Other comprehensive income (loss) before reclassifications			(7.9)	5.5
Amounts reclassified from accumulated other comprehensive income (loss)			6.1	1.1
Net current period other comprehensive income (loss)			(1.8)	6.6
Balance, as of the end of the period	(1.9)	(4.2)	(1.9)	(4.2)
Accumulated net unrealized gains (losses) on cash flow hedges
Accumulated other comprehensive income [Line Items]
Balance, as of the beginning of the period			(104.5)	57.8
Cumulative effect of new accounting principle					0.0
Balance, as of January 1, 2018	4.5	(104.5)	(104.5)	57.8	(104.5)
Other comprehensive income (loss) before reclassifications			58.3	(176.5)
Amounts reclassified from accumulated other comprehensive income (loss)			50.7	14.2
Net current period other comprehensive income (loss)			109.0	(162.3)
Balance, as of the end of the period	4.5	(104.5)	4.5	(104.5)
Accumulated unfunded status of postretirement benefit plans
Accumulated other comprehensive income [Line Items]
Balance, as of the beginning of the period			(36.8)	(32.7)
Cumulative effect of new accounting principle					0.0
Balance, as of January 1, 2018	(36.6)	(33.2)	(36.8)	(32.7)	(36.8)
Other comprehensive income (loss) before reclassifications			0.2	(0.5)
Amounts reclassified from accumulated other comprehensive income (loss)			0.0	0.0
Net current period other comprehensive income (loss)			0.2	(0.5)
Balance, as of the end of the period	(36.6)	(33.2)	(36.6)	(33.2)
Accumulated translation adjustment
Accumulated other comprehensive income [Line Items]
Balance, as of the beginning of the period			(175.5)	(334.2)
Cumulative effect of new accounting principle					0.0
Balance, as of January 1, 2018	(214.3)	(187.5)	(175.5)	(334.2)	(175.5)
Other comprehensive income (loss) before reclassifications			(38.8)	146.7
Amounts reclassified from accumulated other comprehensive income (loss)			0.0	0.0
Net current period other comprehensive income (loss)			(38.8)	146.7
Balance, as of the end of the period	$ (214.3)	$ (187.5)	$ (214.3)	$ (187.5)
Accounting Standards Update 2016-01
Accumulated other comprehensive income [Line Items]
Cumulative effect of new accounting principle					1.5
Accounting Standards Update 2016-01 \| Accumulated net unrealized gains (losses) on securities available-for-sale
Accumulated other comprehensive income [Line Items]
Cumulative effect of new accounting principle					$ 1.5

Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) (Details 1) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income (expense), net	$ 115.1	$ (44.0)	$ 39.6	$ (150.6)
Income tax expense (benefit)	369.8	383.8	956.0	892.6
Product revenues	(3,439.0)	(3,077.8)	(9,926.6)	(8,966.9)
Net income attributable to Biogen Inc.	1,444.4	1,226.1	3,483.9	2,836.5
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income attributable to Biogen Inc.	(8.7)	(18.8)	(56.8)	(15.3)
Accumulated net unrealized gains (losses) on securities available-for-sale \| Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income (expense), net	(0.1)	(0.9)	(7.7)	(1.7)
Income tax expense (benefit)	0.0	0.3	1.6	0.6
Accumulated net unrealized gains (losses) on cash flow hedges \| Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income (expense), net	0.1	0.1	0.2	0.2
Income tax expense (benefit)	0.0	0.0	0.2	0.0
Product revenues	(8.4)	(18.8)	(51.7)	(15.1)
Operating expenses	$ 0.3	$ 0.5	$ 0.6	$ 0.7

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Earnings per Share (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net income attributable to Biogen Inc	$ 1,444.4	$ 1,226.1	$ 3,483.9	$ 2,836.5
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	201.4	211.4	206.6	213.0
Effect of dilutive securities:
Dilutive potential common shares	0.5	0.4	0.4	0.3
Shares used in calculating diluted earnings per share	201.9	211.8	207.0	213.3
Stock options and employee stock purchase plan
Effect of dilutive securities:
Stock units	0.0	0.1	0.0	0.0
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	0.4	0.2	0.3	0.2
Market stock units
Effect of dilutive securities:
Stock units	0.1	0.1	0.1	0.1
Performance stock units settled in stock
Effect of dilutive securities:
Stock units	0.0	0.0	0.0	0.0

Share-Based Payments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
Share-Based Payments (Details) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Expense included in consolidated statements of income
Subtotal	$ 50.4	$ 43.1	$ 143.6	$ 127.6
Capitalized share-based compensation costs	(3.5)	(2.5)	(9.7)	(7.6)
Share-based compensation expense included in total costs and expenses	46.9	40.6	133.9	120.0
Income tax effect	(7.7)	(10.9)	(21.8)	(31.8)
Research and development
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	21.0	19.5	60.5	55.7
Selling, general and administrative
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	29.4	23.6	83.1	71.9
Total share-based compensation expense, net of tax
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	$ 39.2	$ 29.7	$ 112.1	$ 88.2

Share-Based Payments (Details 1) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
Share-Based Payments (Details 1) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Summary of share based compensation expense associated with different programs [Abstract]
Subtotal	$ 50.4	$ 43.1	$ 143.6	$ 127.6
Capitalized share-based compensation costs	(3.5)	(2.5)	(9.7)	(7.6)
Share-based compensation expense included in total costs and expenses	46.9	40.6	133.9	120.0
Market stock units
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	7.4	4.5	20.6	17.1
Time-vested restricted stock units
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	29.9	26.7	96.6	81.2
Cash settled performance shares
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	5.8	7.0	9.4	13.0
Performance units
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	3.0	3.2	4.2	8.9
Performance stock units settled in stock
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	1.3	0.0	3.4	0.0
Performance stock units settled in cash
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	1.1	0.0	1.5	0.0
Employee stock purchase plan
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	$ 1.9	$ 1.7	$ 7.9	$ 7.4

Share-based Payments Share-based Payments (Details Textual)	9 Months Ended
	Sep. 30, 2018 $ / shares shares
Performance stock units settled in stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of earnings of the target number of units granted based on actual stock performance (Minimum)	0.00%
Percentage of earnings of the target number of units granted based on actual stock performance (Maximum)	200.00%
Number of shares granted	67,000
Weighted average grant date fair value \| $ / shares	$ 317.09
Performance stock units settled in cash
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of earnings of the target number of units granted based on actual stock performance (Minimum)	0.00%
Percentage of earnings of the target number of units granted based on actual stock performance (Maximum)	200.00%
Number of shares granted	45,000
Number of days for calculation of average closing stock price	30 days

Income Taxes (Details)	3 Months Ended		9 Months Ended
Income Taxes (Details)	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reconciliation between the U.S. federal statutory tax rate and effective tax rate
Statutory rate	21.00%	35.00%	21.00%	35.00%
State taxes	0.60%	0.70%	0.70%	0.60%
Taxes on foreign earnings	0.80%	(11.40%)	0.10%	(11.40%)
Credits and net operating loss utilization	(1.00%)	(0.70%)	(0.80%)	(0.80%)
Purchased intangible assets	0.30%	1.20%	0.50%	1.30%
Manufacturing deduction	0.00%	(2.00%)	0.00%	(2.10%)
Permanent items	0.30%	0.60%	0.30%	0.70%
Tax reform	(0.60%)	(0.00%)	(0.30%)	(0.00%)
Other	(1.00%)	0.40%	(0.20%)	0.60%
Effective tax rate	20.40%	23.80%	21.30%	23.90%

Income Taxes (Details Textual) - USD ($) $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
Income Taxes (Details Textual) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Jan. 01, 2018
Income Tax Contingency [Line Items]
Statutory rate	21.00%		35.00%	21.00%	35.00%
Income tax expense	$ 369.8		$ 383.8	$ 956.0	$ 892.6
2017 Tax Act
Income Tax Contingency [Line Items]
Statutory rate				21.00%		35.00%
Income tax expense		$ 1,200.0
Transition toll tax liabilities	34.6	989.6		$ 34.6		$ 989.6
Remeasurement of our deferred tax balances in our provisional estimate	5.1	184.0		12.7
Tax expense reflected for other aspects of 2017 Tax Act	11.0
Deferred tax liabilities	695.9	$ 989.6		695.9		$ 989.6
Payment for Transition Toll Tax				85.0
Deferred Tax Liabilities, Other	150.0
Foreign tax authority
Income Tax Contingency [Line Items]
Income tax payment, including penalties and interest	$ 60.0			$ 60.0
Foreign earnings in the form of cash and cash equivalents \| 2017 Tax Act
Income Tax Contingency [Line Items]
Transition toll tax repatriation tax rate		15.50%				15.50%
Other foreign earnings \| 2017 Tax Act
Income Tax Contingency [Line Items]
Transition toll tax repatriation tax rate		8.00%				8.00%
Accounting Standards Update 2016-16 \| Deferred tax asset
Income Tax Contingency [Line Items]
Cumulative effect of new accounting principle							$ 2,000.0
Accounting Standards Update 2016-16 \| Deferred tax liability
Income Tax Contingency [Line Items]
Cumulative effect of new accounting principle							1,500.0
Accounting Standards Update 2016-16 \| Retained earnings
Income Tax Contingency [Line Items]
Cumulative effect of new accounting principle							$ 500.0

Other Consolidated Financial Statement Detail (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Other Income (Expense), Net
Interest income	$ 26.0	$ 20.6	$ 81.4	$ 54.2
Interest expense	(49.0)	(61.8)	(151.2)	(188.8)
Gain (loss) on investments, net	141.1	(4.0)	132.0	(15.0)
Foreign exchange gains (losses), net	0.2	6.7	(13.8)	8.4
Other, net	(3.2)	(5.5)	(8.8)	(9.4)
Total other income (expense), net	115.1	$ (44.0)	39.6	$ (150.6)
Accrued Expenses and Other
Revenue-related reserves for discounts and allowances	935.8		935.8		$ 761.6
Current portion of contingent consideration obligations	381.5		381.5		844.6
Employee compensation and benefits	274.1		274.1		297.7
Royalties and licensing fees	226.3		226.3		206.7
Construction in progress	196.6		196.6		159.7
Collaboration expenses	101.5		101.5		183.7
Other	640.0		640.0		636.9
Total accrued expenses and other	2,578.5		2,578.5		2,901.3
Component of accrued expenses and other
Accrued Expenses and Other
Revenue-related reserves for discounts and allowances	$ 758.5		$ 758.5		$ 572.0

Other Consolidated Financial Statement Detail Other Consolidated Financial Statement (Details Textual) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Other Assets [Abstract]
Other current assets	$ 848.3	$ 962.0
Prepaid taxes	431.8	657.6
Other long-term liabilities	1,511.8	1,628.7
Accrued income taxes	$ 786.9	$ 979.8

Collaborative and Other Relationships Collaborative and Other Relationships - Collaborations (Details) - USD ($) shares in Millions, $ in Millions	1 Months Ended	3 Months Ended				9 Months Ended
	Jan. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Dec. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Jun. 05, 2018
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development		$ 507.9			$ 446.4		$ 1,985.6	$ 1,666.0
Inventory charges		460.8			370.0		1,327.8	1,120.8
Total revenues		3,439.0			3,077.8		9,926.6	8,966.9
Collaboration profit (loss) sharing		47.5			35.2		129.2	82.5
ZINBRYTA
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development				$ 12.8
Inventory charges				$ 2.4
Total revenues		0.0			14.2		1.4	41.0
OCREVUS
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty operating profit threshold for highest royalty rate percentage							$ 900.0
Reduction in royalty rate							50.00%
Period of collaboration agreement							11 years
Revenues from anti-CD20 therapeutic programs
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total revenues		511.7			406.5		$ 1,445.3	1,144.2
Ionis Pharmaceuticals
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Term of collaboration agreement			10 years
Total payment to enter collaboration agreement									$ 1,000.0
Upfront payment for collaboration agreement									$ 375.0
Investment in common stock, shares purchased									11.5
Purchase of common stock									$ 625.0
Prepaid research and development services			$ 50.9
Research and development			$ 324.1
Investment in common stock, value									462.9
Premium on purchase of common stock									$ 162.1
AbbVie
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Biogen share of co-promotion profits or losses							50.00%
Eisai \| BAN2401 and Elenbecestat
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Expense incurred by the collaboration		64.9			38.3		$ 176.0	105.0
Expense reflected within statements of income		32.5			19.2		88.0	52.5
Reimbursement of aducanumab \| Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of future development costs related to Eisai	45.00%					15.00%
U.S. \| ZINBRYTA
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total revenues		0.0			0.0		0.0	0.0
E.U. and Canada \| AbbVie
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration profit (loss) sharing		0.2			0.7		2.0	2.0
Rest of world \| ZINBRYTA
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total revenues		0.0			14.2		$ 1.4	41.0
Rest of world \| OCREVUS
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalties percentage to be received on sale of product							3.00%
Minimum \| OCREVUS
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalties percentage to be received on sale of product							13.50%
Minimum \| Ionis Pharmaceuticals
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Estimated additional payments upon achievement of milestones							$ 125.0
Maximum \| OCREVUS
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalties percentage to be received on sale of product							24.00%
Maximum \| Ionis Pharmaceuticals
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Estimated additional payments upon achievement of milestones							$ 270.0
Research and development \| Eisai \| Aducanumab
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Expense incurred by the collaboration		64.8			73.3		204.8	202.2
Expense reflected within statements of income		55.1			73.3		183.6	202.2
Selling, general and administrative \| Eisai \| Aducanumab
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Expense incurred by the collaboration		12.1			5.1		33.3	14.9
Expense reflected within statements of income		5.1			5.1		19.0	14.9
AbbVie \| Collaborative arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total revenues		(0.7)			(2.8)		(7.9)	(12.6)
AbbVie \| U.S. \| Collaborative arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total revenues		(0.7)			(2.8)		(7.9)	(12.6)
Genentech
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA		358.0			325.1		1,066.6	996.1
Other revenues from anti-CD20 therapeutic programs		$ 153.7			$ 81.4		$ 378.7	$ 148.1

Collaborative and Other Relationships Collaborative and Other Relationships - Equity Method Investments (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended
	Feb. 29, 2012	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Schedule of Equity Method Investments [Line Items]
Collaboration profit (loss) sharing		$ 47.5	$ 35.2	$ 129.2	$ 82.5
Total revenues		$ 3,439.0	3,077.8	$ 9,926.6	8,966.9
Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Percentage of stake in entity		5.00%		5.00%
Percentage of stake in entity maximum	49.90%
Payments to acquire additional investment in equity method investment				$ 700.0
Biogen share of co-promotion profits or losses				50.00%
Collaboration profit (loss) sharing		$ 47.7	34.5	$ 131.2	80.5
Collaborative arrangement \| Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Total revenues		$ 48.1	$ 8.8	$ 80.7	$ 23.7

Investments in Variable Interest Entities (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended	9 Months Ended
	Oct. 31, 2017	Jun. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Variable Interest Entity [Line Items]
Payments to Neurimmune			$ 36.9	$ 0.0
Investment in Variable Interest Entities (Textual)
Investment in biotechnology companies that are determined to be unconsolidated variable interest entities			$ 29.2		$ 48.3
Neurimmune
Variable Interest Entity [Line Items]
Payments to Neurimmune	$ 150.0	$ 50.0
Reduction in royalty rate payable on commercial sales	15.00%
Additional reduction in royalty rate payable on commercial sales		5.00%

Litigation Litigation (Details)	9 Months Ended
Litigation Litigation (Details)	Sep. 30, 2018
Brazil
Income Tax Contingency [Line Items]
Brazil tax assessment, including interest and penalties	70.0