Revenues Revenues from Anti-CD20 Therapeutic Programs (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Total revenues from anti-cd20 therapeutic programs	$ 3,671.3	$ 3,600.1	$ 3,616.7	$ 3,489.8	$ 3,526.3	$ 3,439.0	$ 3,356.5	$ 3,131.1	$ 14,377.9	$ 13,452.9	$ 12,273.9
Genentech
Disaggregation of Revenue [Line Items]
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA, including the reimbursement of selling and development expenses									1,542.4	1,431.9	1,316.4
Other revenues from anti-CD20 therapeutic programs									748.0	548.3	242.8
Revenues from anti-cd20 therapeutic programs
Disaggregation of Revenue [Line Items]
Total revenues from anti-cd20 therapeutic programs	$ 600.8	$ 595.8	$ 576.4	$ 517.4	$ 534.9	$ 511.7	$ 490.4	$ 443.2	$ 2,290.4	$ 1,980.2	$ 1,559.2