v3.25.2
Cover Page
Jul. 31, 2025
Cover [Abstract]  
Document Type 8-K
Document Period End Date Jul. 31, 2025
Entity Registrant Name BIOGEN INC.
Entity Incorporation, State or Country Code DE
Entity File Number 0-19311
Entity Tax Identification Number 33-0112644
Entity Address, Address Line One 225 Binney Street
Entity Address, City or Town Cambridge
Entity Address, State or Province MA
Entity Address, Postal Zip Code 02142
City Area Code 617
Local Phone Number 679-2000
Written Communications false
Soliciting Material false
Pre-commencement Tender Offer false
Pre-commencement Issuer Tender Offer false
Title of 12(b) Security Common Stock, $0.0005 par value
Trading Symbol BIIB
Security Exchange Name NASDAQ
Entity Emerging Growth Company false
Entity Central Index Key 0000875045
Amendment Flag false