Investments in Variable Interest Entities (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended					6 Months Ended
	Jun. 30, 2021	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Variable Interest Entity [Line Items]
(Gain) loss on equity method investments							$ 2.6	$ 16.1
Deferred tax assets, net					$ 490.0
Deferred tax assets, unrecognized tax benefit		$ 85.0		$ 390.0			85.0
Investment in biotechnology companies that are determined to be unconsolidated variable interest entities		23.5		24.6			$ 23.5
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			5 years				10 years
(Gain) loss on equity method investments		28.9				$ 40.1	$ 210.6	40.1
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	$ 0.0				$ 100.0	$ 0.0	$ 100.0
Eisai | Regulatory Milestones | Neurimmune | ADUHELM
Variable Interest Entity [Line Items]
Potential future milestone payments commitment to third party					$ 100.0