| Cover Page | Oct. 22, 2019 |
| Cover page. | |
| Document Type | 8-K |
| Document Period End Date | Oct. 22, 2019 |
| 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 |
