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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% |  |

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