Revenues - Revenues from Anti-CD20 Therapeutic Programs (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	$ 2,465.8	$ 2,530.3	$ 7,221.2	$ 7,449.3
Genentech
Disaggregation of Revenue [Line Items]
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA	94.8	98.9	285.3	315.0
Other revenues from anti-CD20 therapeutic programs	4.6	2.9	13.6	10.6
Revenue from anti-CD20 therapeutic programs
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	446.2	420.9	1,284.7	1,253.8
Royalty Attributed To OCREVUS
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	$ 346.8	$ 319.1	$ 985.8	$ 928.2