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Collaborative and Other Relationships - Equity Method Investments (Details) $ in Millions, ₩ in Billions | 1 Months Ended | 3 Months Ended | 9 Months Ended |  |  |  |
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Jun. 30, 2021 USD ($) | Nov. 30, 2018 USD ($) | Nov. 30, 2018 KRW (₩) | Sep. 30, 2021 USD ($) | Jun. 30, 2021 USD ($) | Sep. 30, 2020 USD ($) | Jun. 30, 2020 USD ($) | Mar. 31, 2020 USD ($) | Sep. 30, 2021 USD ($) | Sep. 30, 2020 USD ($) | Sep. 30, 2021 KRW (₩) | Dec. 31, 2020 USD ($) | Dec. 31, 2020 KRW (₩) | Jan. 31, 2020 USD ($) | Nov. 07, 2018 USD ($)
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income (loss) from equity method investments |  |  |  |  |  |  |  |  | $ 17.2 | $ (14.7) |  |  |  |  |
Income tax (benefit) expense |  |  |  | $ (25.9) |  | $ 240.8 |  |  | (390.7) | 979.0 |  |  |  |  |
Collaboration profit sharing |  |  |  | 21.2 |  | 73.0 |  |  | 74.5 | 166.5 |  |  |  |  |
Total revenue |  |  |  | 2,778.9 |  | 3,376.1 |  |  | 8,247.9 | 10,592.0 |  |  |  |  |
ADUHELM |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total revenue |  |  |  | $ 0.3 |  | $ 0.0 |  |  | $ 2.0 | $ 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% |  |  |  |
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 |  |  |  | $ 1.1 |  | $ (13.1) |  |  | $ 17.2 | $ (12.7) |  |  |  |  |
Investment in Samsung Bioepis |  |  |  | 586.1 |  |  |  |  | 586.1 |  | ₩ 692.8 | $ 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 |  |  |  | 71.8 |  | 72.9 |  |  | $ 210.2 | 200.1 |  |  |  |  |
Eisai |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss on research and development contracts terminated with Eisai |  |  |  |  |  |  | $ 45.0 |  |  |  |  |  |  |  |
Income (loss) from equity method investments |  |  |  | 50.6 |  | 33.8 | $ 33.8 |  | 90.7 |  |  |  |  |  |
Additional milestone payment |  |  |  |  |  | 75.0 |  |  |  |  |  |  |  |  |
Eisai | ADUHELM |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income (loss) from equity method investments |  |  |  | 45.0 | $ 45.0 |  |  |  | 45.0 |  |  |  |  |  |
Additional milestone payment | $ 100.0 |  |  | 0.0 |  | $ 0.0 |  |  | 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 |
Additional milestone payment |  |  |  | 15.0 |  |  |  |  |  |  |  |  |  |  |
Accrued milestone payments |  |  |  | 15.0 |  |  |  |  |  |  |  |  |  |  |
Estimated additional payments upon achievement of development and commercial milestones |  |  |  |  |  |  |  |  | 180.0 |  |  |  |  |  |
Contract Option Exercise Fee |  |  |  |  |  |  |  |  | 60.0 |  |  |  |  |  |
Due to Related Parties |  |  |  | $ 125.7 |  |  |  |  | $ 125.7 |  |  | $ 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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