Investments in Variable Interest Entities (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended				9 Months Ended
	Jun. 30, 2021	Sep. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Variable Interest Entity [Line Items]
(Gain) loss on equity method investments						$ 2.6	$ 17.2
Percentage of future development costs related to Eisai						45.00%
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		25.3		24.6		$ 25.3
Neurimmune
Variable Interest Entity [Line Items]
Collaboration agreement term						12 years
Research and development costs, percentage						100.00%
Eisai
Variable Interest Entity [Line Items]
(Gain) loss on equity method investments		3.4			$ 50.6	$ 214.0	90.7
Percentage of future development costs related to Eisai					45.00%
Eisai | ADUHELM
Variable Interest Entity [Line Items]
(Gain) loss on equity method investments			$ 59.0	45.0	$ 45.0	177.0
Additional milestone payment	$ 100.0	$ 0.0			$ 0.0	$ 0.0	$ 100.0
Eisai | Regulatory Milestones | Neurimmune | ADUHELM
Variable Interest Entity [Line Items]
Potential future milestone payments commitment to third party				$ 100.0