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Investment in biotechnology companies that are determined to be unconsolidated variable interest entities.

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Percentage of future development costs related to Eisai

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Potential future milestone payments commitment to third party approximately.

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Reduction in royalty rate payable on commercial sales

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Amount of income (loss) for proportionate share of equity method investee's income (loss).

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Amount of cash outflow to a noncontrolling interest. Includes, but not limited to, reduction of noncontrolling interest ownership. Excludes dividends paid to the noncontrolling interest.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Investments in Variable Interest Entities (Details) - USD ($) $ in Millions	1 Months Ended		3 Months Ended
	May 31, 2018	Oct. 31, 2017	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Variable Interest Entity [Line Items]
Investment in biotechnology companies that are determined to be unconsolidated variable interest entities					$ 12.8
Loss on equity method investment			$ (18.2)	$ (16.8)
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%
Twelve months
Variable Interest Entity [Line Items]
Potential Future Milestone Payments Commitment To Third Party Approximately			$ 150.0
Regulatory Milestones [Member] \| Neurimmune
Variable Interest Entity [Line Items]
Potential Future Milestone Payments Commitment To Third Party Approximately					$ 75.0
Eisai
Variable Interest Entity [Line Items]
Loss on equity method investment			$ 33.8
Percentage of future development costs related to Eisai			45.00%
Aducanumab \| Regulatory Milestones [Member] \| JAPAN
Variable Interest Entity [Line Items]
Potential Future Milestone Payments Commitment To Third Party Approximately			$ 50.0
Neurimmune \| Regulatory Milestones [Member]
Variable Interest Entity [Line Items]
Potential Future Milestone Payments Commitment To Third Party Approximately			$ 100.0