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Investments in Variable Interest Entities (Details) - USD ($) $ in Millions	1 Months Ended			3 Months Ended				9 Months Ended
	Jun. 30, 2021	May 31, 2018	Oct. 31, 2017	Sep. 30, 2021	Jun. 30, 2021	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Variable Interest Entity [Line Items]
Payments to Noncontrolling Interests								$ 94.8	$ 70.9
(Gain) loss on equity method investment								17.2	(14.7)
Deferred tax assets, net				$ 500.0				500.0
Deferred tax assets, gross	$ 875.0				$ 875.0
Deferred tax assets, unrecognized tax benefit	375.0				$ 375.0
Investment in biotechnology companies that are determined to be unconsolidated variable interest entities				17.2				$ 17.2		$ 12.8
Neurimmune
Variable Interest Entity [Line Items]
Collaboration agreement term								12 years
Research and development costs, percentage								100.00%
Payments to Noncontrolling Interests		$ 50.0	$ 150.0
Reduction in royalty rate payable on commercial sales			15.00%
Additional reduction in royalty rate payable on commercial sales		5.00%
Regulatory Milestones | Neurimmune
Variable Interest Entity [Line Items]
Potential future milestone payments commitment to third party							$ 75.0
Eisai
Variable Interest Entity [Line Items]
(Gain) loss on equity method investment				50.6		$ 33.8	$ 33.8	$ 90.7
Percentage of future development costs related to Eisai					45.00%	45.00%	45.00%
Additional milestone payment						$ 75.0
Eisai | ADUHELM
Variable Interest Entity [Line Items]
(Gain) loss on equity method investment				45.0	$ 45.0			45.0
Additional milestone payment	100.0			0.0		$ 0.0		100.0	$ 75.0
Eisai | Regulatory Milestones | Neurimmune | ADUHELM
Variable Interest Entity [Line Items]
Potential future milestone payments commitment to third party	$ 100.0				$ 100.0
Eisai | Regulatory Milestones | JAPAN | ADUHELM
Variable Interest Entity [Line Items]
Potential future milestone payments commitment to third party				50.0				50.0
Eisai | Regulatory Milestones | Europe | ADUHELM
Variable Interest Entity [Line Items]
Potential future milestone payments commitment to third party				$ 50.0				$ 50.0

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