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					6 Months Ended
Inventory - Narrative (Details) - USD ($) $ in Millions	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Inventory [Line Items]
Excess and obsolescence charges related to inventory						$ 305.6	$ 8.3
(Gain) loss on equity method investments						2.6	16.1
Eisai
Inventory [Line Items]
(Gain) loss on equity method investments	$ 28.9				$ 40.1	$ 210.6	$ 40.1
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