Summary of Significant Accounting Policies - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($) payment segment	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Jul. 31, 2023	Apr. 30, 2022 USD ($)	Mar. 31, 2022	Feb. 29, 2012
Property, Plant and Equipment [Line Items]
Interest in subsidiary (less than given percentage)	100.00%
Number of operating segments | segment	1
Number of reportable segment | segment	1
Advertising costs	$ 71.4	$ 54.1	$ 98.7
LEQEMBI Collaboration
Property, Plant and Equipment [Line Items]
Share of non-controling interest recognized	50.00%			50.00%
Samsung Bioepis
Property, Plant and Equipment [Line Items]
Due from joint venture					$ 1,300.0
Number of payments | payment	2
Samsung Bioepis | Payment Due At First Anniversary
Property, Plant and Equipment [Line Items]
Due from joint venture					812.5
Samsung Bioepis | Payment Due At Second Anniversary
Property, Plant and Equipment [Line Items]
Due from joint venture					$ 437.5
Samsung Bioepis
Property, Plant and Equipment [Line Items]
Ownership percentage						49.90%	15.00%
Minimum
Property, Plant and Equipment [Line Items]
Payment terms of accounts receivable arising from product sales	30 days
Minimum | Computer Software and Hardware
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	3 years
Maximum
Property, Plant and Equipment [Line Items]
Payment terms of accounts receivable arising from product sales	90 days
Maximum | Computer Software and Hardware
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	5 years