Inventory - Narrative (Details) - USD ($) $ in Millions	3 Months Ended				12 Months Ended
	Mar. 31, 2022	Dec. 31, 2021	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Inventory [Line Items]
Write-downs on excess, obsolete, unmarketable or other inventory					$ 336.2	$ 167.6	$ 26.6
Inventory		$ 1,351.5			1,365.5	1,351.5
Income (loss) from equity method investments					2.6	34.9	3.3
Eisai
Inventory [Line Items]
Income (loss) from equity method investments				$ 33.8	0.0	45.0	$ 33.8
ADUHELM
Inventory [Line Items]
Write-downs on excess, obsolete, unmarketable or other inventory	$ 275.0	120.0
Inventory		$ 223.0				223.0
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