Equity - Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Income tax expense and effective tax rate	$ 263,600	$ 273,800	$ 135,300
Revenue	9,890,600	9,675,900	9,835,600
Net income attributable to Biogen Inc.	1,292,900	1,632,200	1,161,100
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net income attributable to Biogen Inc.	(45,900)	4,200	(100)
Gains (losses) on securities available for sale | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other (income) expense, net	0	0	(17,000)
Income tax expense and effective tax rate	0	0	3,600
Gains (losses) on cash flow hedges | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other (income) expense, net	(300)	(400)	(300)
Income tax expense and effective tax rate	5,500	(600)	(1,700)
Revenue	(72,400)	18,100	11,600
Operating expense	$ 21,300	$ (12,900)	$ 3,700