Inventory - Narrative (Details) - USD ($) $ in Millions	3 Months Ended					9 Months Ended
	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Inventory [Line Items]
Excess and obsolescence charges related to inventory						$ 321.6	$ 36.5
(Gain) loss on equity method investments						$ 2.6	17.2
Percentage of future development costs related to Eisai						45.00%
Eisai
Inventory [Line Items]
(Gain) loss on equity method investments	$ 3.4				$ 50.6	$ 214.0	$ 90.7
Percentage of future development costs related to Eisai					45.00%
ADUHELM
Inventory [Line Items]
Excess and obsolescence charges related to inventory		$ 275.0	$ 120.0
Inventory				$ 223.0
ADUHELM | Eisai
Inventory [Line Items]
(Gain) loss on equity method investments			$ 59.0	$ 45.0	$ 45.0	177.0
ADUHELM | Eisai | Centers for Medicare and Medicaid Service
Inventory [Line Items]
(Gain) loss on equity method investments						$ 136.0