Collaborative and Other Relationships - Genentech (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	Sep. 30, 2017	Mar. 31, 2016
Revenues from unconsolidated joint business
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
Revenues from unconsolidated joint business
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
GAZYVA
After First GA101 Threshold Date
Until First GAZYVA Threshold Date	39.00%								39.00%
After First Threshold Date and until Second Threshold Date	37.50%								37.50%
After Second Threshold Date	35.00%								35.00%
RITUXAN
Co-promotion profit sharing formula
Until GAZYVA First Non-CLL FDA Approval	40.00%								40.00%
After First GA101 Threshold Date
Until First GAZYVA Threshold Date	39.00%								39.00%		37.50%		39.00%
After First Threshold Date and until Second Threshold Date	37.50%				37.50%				37.50%	37.50%		37.50%
After Second Threshold Date	35.00%								35.00%
Revenues from anti-cd20 therapeutic programs
Revenues from unconsolidated joint business
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