Segment Information - Additional Segment Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue	$ 9,890.6	$ 9,675.9	$ 9,835.6
Cost of sales, excluding amortization and impairment of acquired intangible assets	2,404.2	2,310.4	2,533.4
Research and development:	1,778.6	1,980.3	2,445.4
Selling, general and administrative	2,433.6	2,403.7	2,549.7
Net income attributable to Biogen Inc.	1,292.9	1,632.2	1,161.1
Product revenue, net
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue	7,119.4	7,213.5	7,246.7
Royalty and other revenue
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue	53.5	60.5	51.1
Reportable Segment
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue	9,890.6	9,675.9	9,835.6
Selling, general and administrative	2,433.6	2,403.7	2,549.7
Other segment expense	1,509.5	1,287.8	1,129.4
Net income attributable to Biogen Inc.	1,292.9	1,632.2	1,161.1
Reportable Segment | Research and discovery
Segment Reporting, Asset Reconciling Item [Line Items]
Research and development:	183.2	201.5	212.5
Reportable Segment | Early stage programs
Segment Reporting, Asset Reconciling Item [Line Items]
Research and development:	257.7	286.6	361.0
Reportable Segment | Late stage programs
Segment Reporting, Asset Reconciling Item [Line Items]
Research and development:	232.0	209.7	250.5
Reportable Segment | Marketed products
Segment Reporting, Asset Reconciling Item [Line Items]
Research and development:	412.8	534.7	766.1
Reportable Segment | Service, Other
Segment Reporting, Asset Reconciling Item [Line Items]
Research and development:	692.9	747.8	855.3
Reportable Segment | Product revenue, net
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue	7,119.4	7,213.5	7,246.7
Cost of sales, excluding amortization and impairment of acquired intangible assets	1,587.2	1,604.2	1,787.2
Reportable Segment | Royalty and other revenue
Segment Reporting, Asset Reconciling Item [Line Items]
Cost of sales, excluding amortization and impairment of acquired intangible assets	$ 817.0	$ 706.2	$ 746.2