Revenues - Revenues from Anti-CD20 Therapeutic Programs (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	$ 2,463.0	$ 2,531.8
Genentech
Disaggregation of Revenue [Line Items]
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA	112.5	143.2
Other revenues from anti-CD20 therapeutic programs	3.4	3.9
Revenue from anti-CD20 therapeutic programs
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	399.5	399.4
Royalty Attributed To OCREVUS
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	$ 283.6	$ 252.3