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Collaborative and Other Relationships - Samsung Bioepis (Details) $ in Millions, ₩ in Billions | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |  |  |
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Jan. 31, 2020 USD ($) | Nov. 30, 2018 USD ($) | Nov. 30, 2018 KRW (₩) | Dec. 31, 2013 USD ($) | Feb. 29, 2012 USD ($) | Dec. 31, 2020 USD ($) | Sep. 30, 2020 USD ($) | Jun. 30, 2020 USD ($) | Mar. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Sep. 30, 2019 USD ($) | Jun. 30, 2019 USD ($) | Mar. 31, 2019 USD ($) | Dec. 31, 2020 USD ($) product | Dec. 31, 2019 USD ($) product | Dec. 31, 2018 USD ($) | Dec. 31, 2020 KRW (₩) | Dec. 31, 2019 KRW (₩) | Jun. 30, 2018 | Feb. 29, 2012 KRW (₩)
Collaborative arrangements and non-collaborative arrangement transactions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to increase investment in Samsung Bioepis |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ 676.6 |  |  |  |
Eisai's share of aducanumab milestone expense reflected in collaboration profit sharing in our consolidated statements of income |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 3.3 | $ (77.4) | 0.0 |  |  |  |
Number of ophthalmolgy biosimilar products | product |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 | 2 |  |  |  |  |
Research and Development Expense (Excluding Acquired in Process Cost) |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 3,990.9 | $ 2,280.6 | 2,597.2 |  |  |  |
Collaboration profit (loss) sharing |  |  |  |  |  |  |  |  |  |  |  |  |  | 232.9 | 241.6 | 185.0 |  |  |  |
Revenues |  |  |  |  |  | $ 2,852.6 | $ 3,376.1 | $ 3,681.6 | $ 3,534.3 | $ 3,671.3 | $ 3,600.1 | $ 3,616.7 | $ 3,489.8 | 13,444.6 | 14,377.9 | 13,452.9 |  |  |  |
Samsung Bioepis |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Collaborative arrangements and non-collaborative arrangement transactions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Upfront and milestone payments made to collaborative partner | $ 100.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Research and development expense asset acquired | 63.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Prepaid research and discovery services | $ 37.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated additional payments upon achievement of development and commercial milestones |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 195.0 |  |  |  |  |  |
Term of collaboration agreement |  |  |  | 10 years |  |  |  |  |  |  |  |  |  | 5 years |  |  |  |  |  |
Option exercise fee |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 60.0 |  |  |  |  |  |
Research and Development Expense (Excluding Acquired in Process Cost) |  |  |  | $ 46.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Expected profit share percentage |  |  |  |  |  |  |  |  |  |  |  |  |  | 50.00% |  |  |  |  |  |
Collaboration profit (loss) sharing |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 266.5 | 241.6 | 187.4 |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  |  |  |  | 20.9 | 106.2 | $ 96.4 |  |  |  |
Due from related parties |  |  |  |  |  | 5.1 |  |  |  | 85.0 |  |  |  | 5.1 | 85.0 |  |  |  |  |
Due to related parties |  |  |  |  |  | 99.0 |  |  |  | 100.0 |  |  |  | 99.0 | 100.0 |  |  |  |  |
Samsung Bioepis | Development Milestones |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Collaborative arrangements and non-collaborative arrangement transactions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total aducanumab collaboration third party milestone expense |  |  |  |  |  |  | $ 15.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Samsung Bioepis |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Collaborative arrangements and non-collaborative arrangement transactions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Contributions by third party |  |  |  |  | $ 250.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | ₩ 280.5
Joint venture ownership percentage |  |  |  |  | 85.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment in Samsung Bioepis |  |  |  |  | $ 45.0 | $ 620.2 |  |  |  | $ 580.2 |  |  |  | $ 620.2 | 580.2 |  | ₩ 673.8 | ₩ 670.8 |  | ₩ 49.5
Samsung Bioepis, ownership percentage |  |  |  |  | 15.00% | 49.90% |  |  |  |  |  |  |  | 49.90% |  |  | 49.90% |  |  | 15.00%
Samsung Bioepis, ownership percentage before additional purchase transaction |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5.00% |
Payments to increase investment in Samsung Bioepis |  | $ 676.6 | ₩ 759.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total basis difference |  | $ 675.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net profits on investment |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 5.3 | 79.4 |  |  |  |  |
Eisai's share of aducanumab milestone expense reflected in collaboration profit sharing in our consolidated statements of income |  |  |  |  |  |  |  |  |  |  |  |  |  | 45.3 | 1.2 |  |  |  |  |
Amortization |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 40.0 | $ 78.2 |  |  |  |  |
Total aducanumab collaboration third party milestone expense |  |  |  | 25.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net change in acquired and in-licensed rights and patents |  |  |  | $ 75.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Samsung Bioepis | Inventories |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Collaborative arrangements and non-collaborative arrangement transactions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment basis difference amortization period |  |  |  |  |  |  |  |  |  |  |  |  |  | 1 year 6 months |  |  |  |  |  |
Samsung Bioepis | Completed technology |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Collaborative arrangements and non-collaborative arrangement transactions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment basis difference amortization period |  |  |  |  |  |  |  |  |  |  |  |  |  | 15 years |  |  |  |  |  |

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