Segment Information - Additional Segment Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Segment Reporting, Asset Reconciling Item [Line Items]
Total revenue	$ 2,431.0	$ 2,290.5
Cost of sales, excluding amortization and impairment of acquired intangible assets	629.3	542.2
Research and development:	434.1	445.4
Acquired in-process research and development, upfront and milestone expense	200.7	7.5
Selling, general and administrative	572.5	581.5
Net income attributable to Biogen Inc.	240.5	393.4
Product revenue, net
Segment Reporting, Asset Reconciling Item [Line Items]
Total revenue	1,726.5	1,711.9
Royalty and other revenue
Segment Reporting, Asset Reconciling Item [Line Items]
Total revenue	11.0	29.6
Reportable Segment
Segment Reporting, Asset Reconciling Item [Line Items]
Total revenue	2,431.0	2,290.5
Other segment expense	353.9	320.5
Net income attributable to Biogen Inc.	240.5	393.4
Reportable Segment | Research and discovery
Segment Reporting, Asset Reconciling Item [Line Items]
Research and development:	44.3	47.6
Reportable Segment | Early stage programs
Segment Reporting, Asset Reconciling Item [Line Items]
Research and development:	72.4	84.1
Reportable Segment | Late stage programs
Segment Reporting, Asset Reconciling Item [Line Items]
Research and development:	49.4	32.6
Reportable Segment | Marketed products
Segment Reporting, Asset Reconciling Item [Line Items]
Research and development:	89.5	118.1
Reportable Segment | Other research and development costs
Segment Reporting, Asset Reconciling Item [Line Items]
Research and development:	178.5	163.0
Reportable Segment | Product revenue, net
Segment Reporting, Asset Reconciling Item [Line Items]
Cost of sales, excluding amortization and impairment of acquired intangible assets	462.2	377.7
Reportable Segment | Royalty and other revenue
Segment Reporting, Asset Reconciling Item [Line Items]
Cost of sales, excluding amortization and impairment of acquired intangible assets	$ 167.1	$ 164.5