v3.26.3
Segment Information - Additional Segment Information (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Segment Reporting [Line Items]        
Total revenue $ 2,804.1 $ 2,534.7 $ 8,017.9 $ 7,611.2
Cost of sales, excluding amortization and impairment of acquired intangible assets 748.4 674.4 2,186.3 1,908.7
Research and development: 574.0 436.1 1,642.6 1,269.2
Acquired in-process research and development, upfront and milestone expense 283.8 2.1 481.8 249.4
Selling, general and administrative 752.0 594.8 2,069.0 1,751.1
Net (loss) income attributable to Biogen Inc. (20.6) 466.5 396.4 1,341.8
Product revenue, net        
Segment Reporting [Line Items]        
Total revenue 2,088.7 1,846.9 5,757.4 5,452.1
Royalty and other revenue        
Segment Reporting [Line Items]        
Total revenue 13.8 9.7 33.5 40.3
Reportable Segment        
Segment Reporting [Line Items]        
Total revenue 2,804.1 2,534.7 8,017.9 7,611.2
Other segment expense 466.5 360.8 1,241.8 1,091.0
Net (loss) income attributable to Biogen Inc. (20.6) 466.5 396.4 1,341.8
Reportable Segment | Research and discovery        
Segment Reporting [Line Items]        
Research and development: 54.2 41.0 136.3 131.9
Reportable Segment | Early stage programs        
Segment Reporting [Line Items]        
Research and development: 87.1 59.9 205.4 184.2
Reportable Segment | Late stage programs        
Segment Reporting [Line Items]        
Research and development: 176.0 60.4 398.3 154.4
Reportable Segment | Marketed products        
Segment Reporting [Line Items]        
Research and development: 112.7 107.7 418.7 284.9
Reportable Segment | Other research and development costs        
Segment Reporting [Line Items]        
Research and development: 144.0 167.1 483.9 513.8
Reportable Segment | Product revenue, net        
Segment Reporting [Line Items]        
Cost of sales, excluding amortization and impairment of acquired intangible assets 541.6 384.0 1,602.6 1,261.2
Reportable Segment | Royalty and other revenue        
Segment Reporting [Line Items]        
Cost of sales, excluding amortization and impairment of acquired intangible assets $ 206.8 $ 290.4 $ 583.7 $ 647.5