Revenue - Revenues from Anti-CD20 Therapeutic Programs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Disaggregation of Revenue [Line Items]
Revenue	$ 9,675.9	$ 9,835.6	$ 10,173.4
Royalty revenue on sales of OCREVUS
Disaggregation of Revenue [Line Items]
Revenue	1,339.5	1,266.2	1,136.3
Revenue from anti-CD20 therapeutic programs
Disaggregation of Revenue [Line Items]
Revenue	1,749.9	1,689.6	1,700.5
Genentech
Disaggregation of Revenue [Line Items]
Biogen's share of pre-tax profits in the U.S. for RITUXAN, GAZYVA and LUNSUMIO	392.0	409.4	547.0
Royalty revenue on sales of OCREVUS	$ 18.4	$ 14.0	$ 17.2