Revenues - Revenues from Anti-CD20 Therapeutic Programs (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	$ 2,778.9	$ 3,376.1	$ 8,247.9	$ 10,592.0
Genentech
Disaggregation of Revenue [Line Items]
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA	145.8	275.0	498.7	873.8
Other revenues from anti-CD20 therapeutic programs	269.6	285.1	745.7	685.0
Revenue from anti-CD20 therapeutic programs
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	$ 415.4	$ 560.1	$ 1,244.4	$ 1,558.8