Investments in Variable Interest Entities (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Mar. 31, 2022
Variable Interest Entity [Line Items]
Potential future milestone payments commitment to third party approximately			$ 9,300.0
Income (loss) from equity method investments			2.6	$ 34.9	$ 3.3
Deferred tax asset, net				100.0
Valuation allowance			2,003.3	1,961.3		$ 85.0
Investment in biotechnology companies that are determined to be unconsolidated variable interest entities			27.8	24.6
ADUHELM
Variable Interest Entity [Line Items]
Income (loss) from equity method investments			$ 111.0	30.0
Neurimmune
Variable Interest Entity [Line Items]
Term of collaboration agreement			12 years
Research and development costs, percentage			100.00%
Regulatory Milestones
Variable Interest Entity [Line Items]
Potential future milestone payments commitment to third party approximately			$ 500.0
Regulatory Milestones | Neurimmune
Variable Interest Entity [Line Items]
Potential future milestone payments commitment to third party approximately		$ 75.0
Eisai
Variable Interest Entity [Line Items]
Income (loss) from equity method investments		$ 33.8	$ 0.0	45.0	$ 33.8
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