Revenues - Revenues from Anti-CD20 Therapeutic Programs (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	$ 2,456.0	$ 2,589.1	$ 4,919.0	$ 5,120.9
Genentech
Disaggregation of Revenue [Line Items]
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA	103.6	139.9	216.1	283.1
Other revenues from anti-CD20 therapeutic programs	4.3	4.6	7.7	8.5
Revenue from anti-CD20 therapeutic programs
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	433.4	436.3	832.9	835.7
Royalty Attributed To OCREVUS
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	$ 325.5	$ 291.8	$ 609.1	$ 544.1