Revenue - Revenues from Anti-CD20 Therapeutic Programs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Disaggregation of Revenue [Line Items]
Revenue	$ 9,890.6	$ 9,675.9	$ 9,835.6
Royalty revenue on sales of OCREVUS
Disaggregation of Revenue [Line Items]
Revenue	1,414.9	1,339.5	1,266.2
Revenue from anti-CD20 therapeutic programs
Disaggregation of Revenue [Line Items]
Revenue	1,860.6	1,749.9	1,689.6
Genentech
Disaggregation of Revenue [Line Items]
Biogen's share of pre-tax profits in the U.S. for RITUXAN, GAZYVA and LUNSUMIO	420.2	392.0	409.4
Royalty revenue on sales of OCREVUS	$ 25.5	$ 18.4	$ 14.0