Investments in Variable Interest Entities (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Jun. 30, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Mar. 31, 2023
Variable Interest Entity [Line Items]
Income (loss) from equity method investments		$ 0.0	$ 2.6	$ 34.9
Potential future milestone payments commitment to third party approximately		5,100.0
Deferred tax asset, net			100.0
Valuation allowance		1,278.7	2,003.3		$ 85.0
Investment in biotechnology companies that are determined to be unconsolidated variable interest entities		16.4	27.8
ADUHELM
Variable Interest Entity [Line Items]
Income (loss) from equity method investments		$ 111.0	30.0
Neurimmune
Variable Interest Entity [Line Items]
Research and development costs, percentage		100.00%
Deconsolidation, gain (loss)		$ 3.0
Regulatory Milestones
Variable Interest Entity [Line Items]
Potential future milestone payments commitment to third party approximately		$ 400.0
Eisai
Variable Interest Entity [Line Items]
Income (loss) from equity method investments			0.0	$ 45.0
Eisai | Milestone Payments
Variable Interest Entity [Line Items]
Percentage of future development costs		45.00%
Eisai | ADUHELM
Variable Interest Entity [Line Items]
Income (loss) from equity method investments	$ 45.0		$ 59.0
Eisai | Regulatory Milestones | Neurimmune | ADUHELM
Variable Interest Entity [Line Items]
Potential future milestone payments commitment to third party approximately	$ 100.0