Investments in Variable Interest Entities (Details) - USD ($) $ in Millions	1 Months Ended		3 Months Ended		6 Months Ended
	May 31, 2018	Oct. 31, 2017	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Variable Interest Entity [Line Items]
Payments to Noncontrolling Interests					$ 0.0	$ (4.3)
Investment in biotechnology companies that are determined to be unconsolidated variable interest entities			$ 22.4		22.4		$ 22.7
Eisai's share of aducanumab milestone expense reflected in collaboration profit sharing in our condensed consolidated statements of income			15.1	$ (16.3)	0.4	$ (45.0)
Neurimmune
Variable Interest Entity [Line Items]
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 [Member] | Neurimmune
Variable Interest Entity [Line Items]
Potential Future Milestone Payments Commitment To Third Party Approximately							75.0
Regulatory Approval Milestone [Member] | Neurimmune
Variable Interest Entity [Line Items]
Potential Future Milestone Payments Commitment To Third Party Approximately							$ 100.0
Eisai
Variable Interest Entity [Line Items]
Eisai's share of aducanumab milestone expense reflected in collaboration profit sharing in our condensed consolidated statements of income			$ 33.8		$ 33.8
Percentage of future development costs related to Eisai					45.00%