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Collaborative and Other Relationships - Equity Method Investments (Details) $ in Millions, ₩ in Billions | 1 Months Ended | 3 Months Ended | 6 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Jun. 30, 2021 USD ($) | Nov. 30, 2018 USD ($) | Nov. 30, 2018 KRW (₩) | Jun. 30, 2021 USD ($) | Jun. 30, 2020 USD ($) | Mar. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Jun. 30, 2021 USD ($) | Jun. 30, 2020 USD ($) | Jun. 30, 2021 KRW (₩) | Dec. 31, 2020 USD ($) | Dec. 31, 2020 KRW (₩) | Jan. 31, 2020 USD ($) | Nov. 07, 2018 USD ($)
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss on equity method investment |  |  |  |  |  |  |  | $ (16.1) | $ 1.6 |  |  |  |  |
Income tax (benefit) expense |  |  |  | $ (409.1) | $ 446.1 |  |  | (364.9) | 738.2 |  |  |  |  |
Collaboration profit sharing |  |  |  | (15.2) | 21.8 |  |  | 53.3 | 93.5 |  |  |  |  |
Total revenue |  |  |  | 2,775.0 | 3,681.6 |  |  | 5,469.0 | 7,215.9 |  |  |  |  |
ADUHELM |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total revenue |  |  |  | $ 1.6 | $ 0.0 |  |  | $ 1.6 | $ 0.0 |  |  |  |  |
Samsung Bioepis |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of stake in entity |  |  |  |  | 5.00% |  |  |  | 5.00% |  |  |  |  |
Percentage of stake in entity maximum | 49.90% |  |  | 49.90% |  |  |  | 49.90% |  | 49.90% |  |  |  |
Payments to acquire additional investment in equity method investment |  | $ 676.6 | ₩ 759.5 |  |  |  | $ 676.6 |  |  |  |  |  |  |
Amortization of basis differences |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 675.0
Loss recorded on Samsung Bioepis joint venture |  |  |  | $ 34.3 | $ 15.1 |  |  | $ 16.1 | $ 0.4 |  |  |  |  |
Investment in Samsung Bioepis | $ 612.8 |  |  | 612.8 |  |  |  | 612.8 |  | ₩ 691.6 | $ 620.2 | ₩ 673.8 |  |
Income tax (benefit) expense |  |  |  | 31.2 |  |  |  | $ 31.2 |  |  |  |  |  |
Biogen share of co-promotion profits or losses |  |  |  |  |  |  |  | 50.00% |  |  |  |  |  |
Collaboration profit sharing |  |  |  | 69.9 | 55.4 |  |  | $ 138.4 | 127.2 |  |  |  |  |
Eisai |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss on research and development contracts terminated with Eisai |  |  |  |  |  | $ 45.0 |  |  |  |  |  |  |  |
Loss on equity method investment |  |  |  | (40.1) | (33.8) |  |  |  |  |  |  |  |  |
Additional milestone payment |  |  |  |  | 75.0 |  |  |  |  |  |  |  |  |
Eisai | ADUHELM |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss on equity method investment |  |  |  | (45.0) |  |  |  | (45.0) |  |  |  |  |  |
Additional milestone payment | 100.0 |  |  | 100.0 | $ 75.0 |  |  | 100.0 | $ 75.0 |  |  |  |  |
Samsung Bioepis |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Upfront and milestone payments made to collaborative partner |  |  |  |  |  | 100.0 |  |  |  |  |  |  |  |
Research and development expense |  |  |  |  |  | 63.0 |  |  |  |  |  |  |  |
Prepaid research and development expenses |  |  |  |  |  |  |  |  |  |  |  |  | $ 37.0 |
Estimated additional payments upon achievement of development and commercial milestones |  |  |  |  |  |  |  | 195.0 |  |  |  |  |  |
Additional milestone payment |  |  |  | 15.0 |  |  |  |  |  |  |  |  |  |
Contract Option Exercise Fee |  |  |  |  |  | $ 60.0 |  |  |  |  |  |  |  |
Due to Related Parties | $ 158.7 |  |  | $ 158.7 |  |  |  | $ 158.7 |  |  | $ 99.0 |  |  |
Inventory | Samsung Bioepis |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment basis difference amortization period |  |  |  |  |  |  |  | 1 year 6 months |  |  |  |  |  |
Developed technology | Samsung Bioepis |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment basis difference amortization period |  |  |  |  |  |  |  | 15 years |  |  |  |  |  |

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