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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
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-Topic 230
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
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-Publisher FASB
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
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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	Mar. 31, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Dec. 31, 2022
Inventory [Line Items]
Excess and obsolescence charges related to inventory			$ 36.8	$ 305.6
(Gain) loss on equity method investments			$ 0.0	$ 2.6
Percentage of future development costs related to Eisai			45.00%
Inventory			$ 1,399.3		$ 1,365.5
Eisai
Inventory [Line Items]
Percentage of future development costs related to Eisai		45.00%
ADUHELM
Inventory [Line Items]
Excess and obsolescence charges related to inventory	$ 275.0	$ 275.0
ADUHELM \| Eisai \| Centers for Medicare and Medicaid Service
Inventory [Line Items]
(Gain) loss on equity method investments			$ 136.0