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Investments in Variable Interest Entities (Details) - USD ($) $ in Millions | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended | 24 Months Ended
--- | --- | --- | --- | --- | ---
May 31, 2018 | Oct. 31, 2017 | Jun. 30, 2020 | Dec. 31, 2018 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2020
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |
Payment to Neurimmune |  |  |  |  | $ 4.6 | $ 0.0 | $ 0.0 |
Eisai's share of aducanumab milestone expense reflected in collaboration profit sharing in our consolidated statements of income |  |  |  |  | 3.3 | (77.4) | 0.0 |
Potential future milestone payments commitment to third party approximately |  |  |  |  | 10,200.0 |  |  | $ 10,200.0
Investment in biotechnology companies that are determined to be unconsolidated variable interest entities |  |  |  |  | 12.8 | 22.7 |  | 12.8
Twelve months |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |
Potential future milestone payments commitment to third party approximately |  |  |  |  | $ 200.0 |  |  | 200.0
Neurimmune |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |
Term of collaboration agreement |  |  |  |  | 12 years |  |  |
Research and development costs, percentage |  |  |  |  | 100.00% |  |  |
Payment to Neurimmune | $ 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 |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |
Potential future milestone payments commitment to third party approximately |  |  |  |  | $ 1,300.0 |  |  | $ 1,300.0
Eisai |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |
Total aducanumab collaboration third party milestone expense |  |  | $ 75.0 |  | 75.0 | 0.0 | 0.0 |
Eisai's share of aducanumab milestone expense reflected in collaboration profit sharing in our consolidated statements of income |  |  |  |  | $ 33.8 | $ 0.0 | $ 0.0 |
Percentage of future development costs related to Eisai |  |  |  | 15.00% | 45.00% |  |  | 45.00%
Neurimmune | Regulatory Milestones |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |
Potential future milestone payments commitment to third party approximately |  |  |  |  | $ 100.0 |  |  | $ 100.0
Aducanumab | Regulatory Milestones | European Union |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |
Potential future milestone payments commitment to third party approximately |  |  |  |  | 50.0 |  |  | 50.0
Aducanumab | Regulatory Milestones | JAPAN |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |
Potential future milestone payments commitment to third party approximately |  |  |  |  | $ 50.0 |  |  | $ 50.0

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