Inventory - Narrative (Details) - USD ($) $ in Millions	3 Months Ended			12 Months Ended
	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Inventory [Line Items]
Write-downs on excess, obsolete, unmarketable or other inventory				$ 124.4	$ 336.2	$ 167.6
Income (loss) from equity method investments				$ 0.0	2.6	34.9
Eisai
Inventory [Line Items]
Income (loss) from equity method investments					0.0	$ 45.0
Eisai | Milestone Payments
Inventory [Line Items]
Percentage of future development costs				45.00%
ADUHELM
Inventory [Line Items]
Write-downs on excess, obsolete, unmarketable or other inventory		$ 275.0	$ 120.0
Income (loss) from equity method investments				$ 111.0	30.0
ADUHELM | Eisai
Inventory [Line Items]
Income (loss) from equity method investments	$ 45.0				$ 59.0
ADUHELM | Centers for Medicare and Medicaid Service | Eisai
Inventory [Line Items]
Income (loss) from equity method investments				$ 136.0