Revenue - Revenues from Anti-CD20 Therapeutic Programs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Disaggregation of Revenue [Line Items]
Revenue	$ 10,173.4	$ 10,981.7	$ 13,444.6
Royalty Attributed To OCREVUS
Disaggregation of Revenue [Line Items]
Revenue	1,136.3	991.7	845.4
Revenue from anti-CD20 therapeutic programs
Disaggregation of Revenue [Line Items]
Revenue	1,700.5	1,658.5	1,977.8
Genentech
Disaggregation of Revenue [Line Items]
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA	547.0	647.7	1,080.2
Royalty revenue on sales of OCREVUS	$ 17.2	$ 19.1	$ 52.2