v3.19.1
Document and Entity Information - shares
3 Months Ended
Mar. 31, 2019
Apr. 19, 2019
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 Mar. 31, 2019  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q1  
Amendment Flag false  
Entity Common Stock, Shares Outstanding   193,893,397