Revenues Reserves for Discounts and Allowances (Details 1) $ in Millions	6 Months Ended
	Jun. 30, 2026 USD ($)
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	$ 1,210.8
Apellis acquisition	52.0
Current Provisions Relating To Sales In Current Year	1,805.3
Adjustments Relating To Prior Years	(57.5)
Payments/Returns Relating To Sales in Current Year	(1,061.2)
Payments/Returns Relating To Sales in Prior Year	(706.4)
Ending Balance	1,243.0
Discounts
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	115.7
Apellis acquisition	13.3
Current Provisions Relating To Sales In Current Year	439.8
Adjustments Relating To Prior Years	(2.0)
Payments/Returns Relating To Sales in Current Year	(316.1)
Payments/Returns Relating To Sales in Prior Year	(129.0)
Ending Balance	121.7
Contractual adjustments
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	1,046.1
Apellis acquisition	32.3
Current Provisions Relating To Sales In Current Year	1,352.1
Adjustments Relating To Prior Years	(69.2)
Payments/Returns Relating To Sales in Current Year	(745.0)
Payments/Returns Relating To Sales in Prior Year	(549.5)
Ending Balance	1,066.8
Returns
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	49.0
Apellis acquisition	6.4
Current Provisions Relating To Sales In Current Year	13.4
Adjustments Relating To Prior Years	13.7
Payments/Returns Relating To Sales in Current Year	(0.1)
Payments/Returns Relating To Sales in Prior Year	(27.9)
Ending Balance	$ 54.5

BIOGEN INC. 10-Q