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Inventory - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Mar. 31, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Inventory [Line Items]
Excess and obsolescence charges related to inventory			$ 62.1	$ 321.6
(Gain) loss on equity method investments			$ 0.0	$ 2.6
Percentage of future development costs related to Eisai			45.00%
Inventory			$ 3,038.3		$ 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

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Inventory, Net Current and Noncurrent

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- Definition

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

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 28
-Subparagraph (b)
-SubTopic 10
-Topic 230
-Publisher FASB
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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-Section S99
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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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-Name Accounting Standards Codification
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Amount of loss from reductions in inventory due to subsequent measurement adjustments, including, but not limited to, physical deterioration, obsolescence, or changes in price levels.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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