Collaborative and Other Relationships - Equity Method Investments (Details) ₩ in Millions, $ in Millions	1 Months Ended		3 Months Ended					9 Months Ended
	Apr. 30, 2022 USD ($)	Dec. 31, 2019	Sep. 30, 2022 USD ($)	Mar. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Sep. 30, 2021 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2022 KRW (₩)	Jun. 30, 2022 KRW (₩)	Jan. 31, 2022 USD ($)	Dec. 31, 2021 KRW (₩)	Jan. 31, 2020 USD ($)	Nov. 07, 2018 USD ($)
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity method investments								$ 2.6	$ 17.2
Collaboration profit (loss) sharing			$ 45.3			$ 21.2		(42.6)	74.5
ADUHELM
Schedule of Equity Method Investments [Line Items]
Research and development expense								$ 300.0
Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Percentage of stake in entity maximum			49.90%					49.90%		49.90%
Amortization of basis differences															$ 675.0
Loss recorded on Samsung Bioepis joint venture						1.1		$ 2.6	17.2
Income (loss) from equity method investments						8.9		17.0	39.5
Investment in Samsung Bioepis					$ 599.9					₩ 0.0			₩ 713,300.0
Collaboration profit (loss) sharing			$ 48.7			71.8		171.4	210.2
Eisai
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity method investments			3.4			50.6		214.0	$ 90.7
Due to Related Parties			81.7		46.5			81.7
Eisai | ADUHELM
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity method investments				$ 59.0	45.0	$ 45.0		$ 177.0
Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Proceeds from divestiture of interest in joint venture	$ 1,000.0
Investment in Samsung Bioepis | ₩											₩ 581.6
Payables to divestiture of interest in joint venture												$ 1,300.0
Equity Method Investments, Expected Profit Share		45.00%						50.00%
Upfront and milestone payments made to collaborative partner							$ 100.0
Research and development expense							$ 63.0
Prepaid research and development expenses														$ 37.0
Accrued milestone payments			15.0
Estimated additional payments upon achievement of development and commercial milestones								$ 180.0
Contract Option Exercise Fee								60.0
Due to Related Parties			$ 53.8		$ 148.7			$ 53.8
Samsung Bioepis | Payment Due At First Anniversary
Schedule of Equity Method Investments [Line Items]
Payables to divestiture of interest in joint venture												812.5
Samsung Bioepis | Payment Due At Second Anniversary
Schedule of Equity Method Investments [Line Items]
Payables to divestiture of interest in joint venture												$ 437.5
Inventory | Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Equity method investment basis difference amortization period								1 year 6 months
Developed technology | Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Equity method investment basis difference amortization period								15 years