Revenues - Revenues from Anti-CD20 Therapeutic Programs (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	$ 2,852.6	$ 3,376.1	$ 3,681.6	$ 3,534.3	$ 3,671.3	$ 3,600.1	$ 3,616.7	$ 3,489.8	$ 13,444.6	$ 14,377.9	$ 13,452.9
Revenues from anti-CD20 therapeutic programs
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	$ 419.0	$ 560.1	$ 478.3	$ 520.4	$ 600.8	$ 595.8	$ 576.4	$ 517.4	1,977.8	2,290.4	1,980.2
Genentech
Disaggregation of Revenue [Line Items]
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA									1,080.2	1,542.4	1,431.9
Other revenues from anti-CD20 therapeutic programs									$ 897.6	$ 748.0	$ 548.3