v3.26.3
Revenues - Revenues from Anti-CD20 Therapeutic Programs (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Disaggregation of Revenue [Line Items]        
Total revenue $ 2,804.1 $ 2,534.7 $ 8,017.9 $ 7,611.2
Total revenues from anti-CD20 therapeutic programs 2,804.1 2,534.7 8,017.9 7,611.2
Genentech        
Disaggregation of Revenue [Line Items]        
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA 118.3 101.0 338.7 292.4
Other revenues from anti-CD20 therapeutic programs 9.1 6.5 22.7 18.0
Royalty Attributed To OCREVUS        
Disaggregation of Revenue [Line Items]        
Total revenue 369.9 386.4 1,068.5 1,029.0
Total revenues from anti-CD20 therapeutic programs 369.9 386.4 1,068.5 1,029.0
Revenue from anti-CD20 therapeutic programs        
Disaggregation of Revenue [Line Items]        
Total revenue 497.3 493.9 1,429.9 1,339.4
Total revenues from anti-CD20 therapeutic programs $ 497.3 $ 493.9 $ 1,429.9 $ 1,339.4