Investments in Variable Interest Entities (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended				12 Months Ended
	Jun. 30, 2021	Mar. 31, 2022	Dec. 31, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2021
Variable Interest Entity [Line Items]
(Gain) loss on equity method investments		$ (3.3)			$ (18.2)
Deferred tax assets, net	$ 490.0			$ 490.0
Deferred tax assets, unrecognized tax benefit		85.0	$ 390.0			$ 390.0
Investment in biotechnology companies that are determined to be unconsolidated variable interest entities		$ 24.1	24.6			$ 24.6
Neurimmune
Variable Interest Entity [Line Items]
Collaboration agreement term		12 years
Research and development costs, percentage		100.00%
Eisai
Variable Interest Entity [Line Items]
Collaboration agreement term		10 years				5 years
(Gain) loss on equity method investments		$ 181.7
Percentage of future development costs related to Eisai		45.00%			45.00%
Eisai | ADUHELM
Variable Interest Entity [Line Items]
(Gain) loss on equity method investments		$ 160.0	$ 59.0	45.0	$ 45.0
Additional milestone payment	100.0
Eisai | ADUHELM | JAPAN
Variable Interest Entity [Line Items]
Additional milestone payment	50.0
Eisai | Regulatory Milestones | Neurimmune | ADUHELM
Variable Interest Entity [Line Items]
Potential future milestone payments commitment to third party	$ 100.0			$ 100.0