Revenue - Revenues from Anti-CD20 Therapeutic Programs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Disaggregation of Revenue [Line Items]
Revenue	$ 9,835.6	$ 10,173.4	$ 10,981.7
Royalty revenue on sales of OCREVUS
Disaggregation of Revenue [Line Items]
Revenue	1,266.2	1,136.3	991.7
Revenue from anti-CD20 therapeutic programs
Disaggregation of Revenue [Line Items]
Revenue	1,689.6	1,700.5	1,658.5
Genentech
Disaggregation of Revenue [Line Items]
Biogen's share of pre-tax profits in the U.S. for RITUXAN, GAZYVA and LUNSUMIO(1)	409.4	547.0	647.7
Royalty revenue on sales of OCREVUS	$ 14.0	$ 17.2	$ 19.1