Amount of income (loss) for proportionate share of equity method investee's income (loss).

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Name:	us-gaap_IncomeLossFromEquityMethodInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Inventory - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
Inventory - Narrative (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021	Jun. 30, 2021	Mar. 31, 2021
Inventory [Line Items]
Excess and obsolescence charges related to inventory	$ 281.5			$ 11.9
(Gain) loss on equity method investments	(3.3)			$ (18.2)
Eisai
Inventory [Line Items]
(Gain) loss on equity method investments	$ 181.7
Percentage of future development costs related to Eisai	45.00%			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	160.0	$ 59.0	$ 45.0	$ 45.0
ADUHELM \| Eisai \| Centers for Medicare and Medicaid Service
Inventory [Line Items]
(Gain) loss on equity method investments	$ 136.0