Inventory - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Inventory [Line Items]
Write-downs on excess, obsolete, unmarketable or other inventory		$ 167.6	$ 26.6	$ 52.2
Inventory	$ 1,351.5	1,351.5	$ 1,068.6
ADUHELM
Inventory [Line Items]
Write-downs on excess, obsolete, unmarketable or other inventory	120.0
Inventory	$ 223.0	$ 223.0