Collaborative and Other Relationships - Revenues from Anti-CD20 Therapeutic Programs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Collaborative arrangements and non-collaborative arrangement transactions
Total revenue from anti-CD20 therapeutic programs	$ 10,981.7	$ 13,444.6	$ 14,377.9
Revenue from anti-CD20 therapeutic programs
Collaborative arrangements and non-collaborative arrangement transactions
Total revenue from anti-CD20 therapeutic programs	1,658.5	1,977.8	2,290.4
Genentech
Collaborative arrangements and non-collaborative arrangement transactions
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA	647.7	1,080.2	1,542.4
Other revenue from anti-CD20 therapeutic programs	$ 1,010.8	$ 897.6	$ 748.0