Subsequent Events	3 Months Ended
Mar. 31, 2022
Subsequent Events [Abstract]
Subsequent Events	SUBSEQUENT EVENTSOn May 3, 2022, Biogen Inc. (the "Company") announced that it had begun a search for a new Chief Executive Officer ("CEO"). The Company has agreed with its current CEO, Michel Vounatsos, that he will continue to serve as CEO during the interim period. Mr. Vounatsos will remain on the Board of Directors of Biogen until he steps down from his CEO position.

The entire disclosure for significant events or transactions that occurred after the balance sheet date through the date the financial statements were issued or the date the financial statements were available to be issued. Examples include: the sale of a capital stock issue, purchase of a business, settlement of litigation, catastrophic loss, significant foreign exchange rate changes, loans to insiders or affiliates, and transactions not in the ordinary course of business.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 855
-URI http://asc.fasb.org/topic&trid=2122774

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 855
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6842918&loc=SL6314017-165662

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