Revenues from Anti-CD20 Therapeutic Programs (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	$ 3,376.1	$ 3,600.1	$ 10,592.0	$ 10,706.6
Genentech
Disaggregation of Revenue [Line Items]
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA	275.0	393.2	873.8	1,161.2
Other revenues from anti-CD20 therapeutic programs	285.1	202.6	685.0	528.4
Revenues from anti-CD20 therapeutic programs
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	$ 560.1	$ 595.8	$ 1,558.8	$ 1,689.6