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Collaborative and Other Relationships - Equity Method Investments (Details) $ in Millions, ₩ in Billions | 1 Months Ended | 3 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Nov. 30, 2018 USD ($) | Nov. 30, 2018 KRW (₩) | Mar. 31, 2021 USD ($) | Mar. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Sep. 30, 2019 USD ($) | Mar. 31, 2021 KRW (₩) | Dec. 31, 2020 USD ($) | Dec. 31, 2020 KRW (₩) | Jan. 31, 2020 USD ($) | Nov. 07, 2018 USD ($)
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Our share of Samsung Bioepis gains (losses) |  |  | $ 18.2 | $ 16.8 |  |  |  |  |  |  |
Collaboration profit (loss) sharing |  |  | 68.5 | 71.8 |  |  |  |  |  |  |
Total revenue |  |  | $ 2,694.0 | $ 3,534.3 |  |  |  |  |  |  |
Samsung Bioepis |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Percentage of stake in entity |  |  |  | 5.00% |  |  |  |  |  |  |
Percentage of stake in entity maximum |  |  | 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 |  |  | $ 18.2 | $ 14.8 |  |  |  |  |  |  |
Investment in Samsung Bioepis |  |  | $ 576.3 |  |  |  | ₩ 653.4 | $ 620.2 | ₩ 673.8 |  |
Biogen share of co-promotion profits or losses |  |  | 50.00% |  |  |  |  |  |  |  |
Collaboration profit (loss) sharing |  |  | $ 68.5 | 71.8 |  |  |  |  |  |  |
Eisai |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Loss on research and development contracts terminated with Eisai |  |  |  |  | $ 45.0 |  |  |  |  |  |
Our share of Samsung Bioepis gains (losses) |  |  | (33.8) |  |  |  |  |  |  |  |
Additional Milestone Payment |  |  | 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 |  |  | $ 188.4 |  |  |  |  | $ 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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- Definition The cash outflow associated with the purchase of or advances to an equity method investments, which are investments in joint ventures and entities in which the entity has an equity ownership interest normally of 20 to 50 percent and exercises significant influence. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3213-108585 + Details
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- Definition The aggregate costs incurred (1) in a planned search or critical investigation aimed at discovery of new knowledge with the hope that such knowledge will be useful in developing a new product or service, a new process or technique, or in bringing about a significant improvement to an existing product or process; or (2) to translate research findings or other knowledge into a plan or design for a new product or process or for a significant improvement to an existing product or process whether intended for sale or the entity's use, during the reporting period charged to research and development projects, including the costs of developing computer software up to the point in time of achieving technological feasibility, and costs allocated in accounting for a business combination to in-process projects deemed to have no alternative future use. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 985 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6501960&loc=d3e128462-111756 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 730 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6420194&loc=d3e21568-108373 + Details
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