Segment Information - Additional Segment Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue	$ 9,675.9	$ 9,835.6	$ 10,173.4
Cost of sales, excluding amortization and impairment of acquired intangible assets	2,310.4	2,533.4	2,278.3
Research and development:	2,041.8	2,462.0	2,231.1
Selling, general and administrative	2,403.7	2,549.7	2,403.6
Net income attributable to Biogen Inc.	1,632.2	1,161.1	3,046.9
Net income	1,632.2	1,161.5	2,961.6
Product revenue, net
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue	7,213.5	7,246.7	7,987.8
Royalty and other revenue
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue	60.5	51.1	67.4
Reportable Segment
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue	9,675.9	9,835.6	10,173.4
Selling, general and administrative	2,403.7	2,549.7	2,403.6
Other segment expense	1,287.8	1,129.4	213.5
Net income attributable to Biogen Inc.	1,632.2	1,161.1	3,046.9
Reportable Segment | Milestone and upfront expense
Segment Reporting, Asset Reconciling Item [Line Items]
Research and development:	61.5	16.6	76.2
Reportable Segment | Research and discovery
Segment Reporting, Asset Reconciling Item [Line Items]
Research and development:	201.5	212.5	289.4
Reportable Segment | Early stage programs
Segment Reporting, Asset Reconciling Item [Line Items]
Research and development:	286.6	361.0	289.9
Reportable Segment | Late stage programs
Segment Reporting, Asset Reconciling Item [Line Items]
Research and development:	209.7	250.5	484.5
Reportable Segment | Marketed products
Segment Reporting, Asset Reconciling Item [Line Items]
Research and development:	534.7	766.1	361.5
Reportable Segment | Service, Other
Segment Reporting, Asset Reconciling Item [Line Items]
Research and development:	747.8	855.3	729.6
Reportable Segment | Product revenue, net
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue	7,213.5	7,246.7	7,987.8
Cost of sales, excluding amortization and impairment of acquired intangible assets	1,604.2	1,787.2	1,504.8
Reportable Segment | Royalty and other revenue
Segment Reporting, Asset Reconciling Item [Line Items]
Cost of sales, excluding amortization and impairment of acquired intangible assets	$ 706.2	$ 746.2	$ 773.5