v3.26.1
Segment Information - Additional Segment Information (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Segment Reporting [Line Items]        
Total revenue $ 2,736.0 $ 2,645.5 $ 5,213.8 $ 5,076.5
Cost of sales, excluding amortization and impairment of acquired intangible assets 776.9 605.0 1,437.9 1,234.3
Research and development: 529.6 399.0 1,068.6 833.1
Acquired in-process research and development, upfront and milestone expense 164.0 46.6 198.0 247.3
Selling, general and administrative 709.7 583.8 1,317.0 1,156.3
Net income attributable to Biogen Inc. 97.5 634.8 417.0 875.3
Product revenue, net        
Segment Reporting [Line Items]        
Total revenue 1,916.4 1,878.7 3,668.7 3,605.2
Royalty and other revenue        
Segment Reporting [Line Items]        
Total revenue 10.0 19.6 19.7 30.6
Reportable Segment        
Segment Reporting [Line Items]        
Total revenue 2,736.0 2,645.5 5,213.8 5,076.5
Other segment expense 458.3 376.3 775.3 730.2
Net income attributable to Biogen Inc. 97.5 634.8 417.0 875.3
Reportable Segment | Research and discovery        
Segment Reporting [Line Items]        
Research and development: 41.6 46.6 82.1 90.9
Reportable Segment | Early stage programs        
Segment Reporting [Line Items]        
Research and development: 49.3 51.9 118.3 124.3
Reportable Segment | Late stage programs        
Segment Reporting [Line Items]        
Research and development: 119.8 44.6 222.3 94.0
Reportable Segment | Marketed products        
Segment Reporting [Line Items]        
Research and development: 151.8 87.7 306.0 177.2
Reportable Segment | Other research and development costs        
Segment Reporting [Line Items]        
Research and development: 167.1 168.2 339.9 346.7
Reportable Segment | Product revenue, net        
Segment Reporting [Line Items]        
Cost of sales, excluding amortization and impairment of acquired intangible assets 582.3 415.0 1,061.0 877.2
Reportable Segment | Royalty and other revenue        
Segment Reporting [Line Items]        
Cost of sales, excluding amortization and impairment of acquired intangible assets $ 194.6 $ 190.0 $ 376.9 $ 357.1