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Financial Instruments - Narrative (Details) shares in Millions, $ in Millions, ₩ in Billions | 1 Months Ended | 12 Months Ended |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Nov. 30, 2018 USD ($) | Nov. 30, 2018 KRW (₩) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2020 KRW (₩) | Nov. 30, 2020 shares | Sep. 30, 2020 shares | Apr. 30, 2020 shares | Dec. 31, 2019 KRW (₩) | Jun. 30, 2018 | Feb. 29, 2012 USD ($) | Feb. 29, 2012 KRW (₩)
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Average maturity of marketable securities, months |  |  | 11 months | 14 months |  |  |  |  |  |  |  |  |
Payments to increase investment in Samsung Bioepis | $ |  |  | $ 0.0 | $ 0.0 | $ 676.6 |  |  |  |  |  |  |  |
Sage Therapeutics |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Dividend yield percentage |  |  | 0.00% |  |  |  |  |  |  |  |  |  |
Sage Therapeutics | Significant Other Observable Inputs (Level 2) |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Investment in common stock, shares purchased (in shares) |  |  |  |  |  |  | 6.2 |  |  |  |  |  |
Denali Therapeutics |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Dividend yield percentage |  |  | 0.00% |  |  |  |  |  |  |  |  |  |
Denali Therapeutics | Significant Other Observable Inputs (Level 2) |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Investment in common stock, shares purchased (in shares) |  |  |  |  |  |  |  | 13.0 |  |  |  |  |
Sangamo Therapeutics |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Investment in common stock, shares purchased (in shares) |  |  |  |  |  |  |  |  | 24.0 |  |  |  |
Dividend yield percentage |  |  | 0.00% |  |  |  |  |  |  |  |  |  |
Strategic Investments |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Strategic investment portfolio | $ |  |  | $ 2,024.6 | 393.9 |  |  |  |  |  |  |  |  |
Samsung Bioepis |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Samsung Bioepis, ownership percentage before additional purchase transaction |  |  |  |  |  |  |  |  |  |  | 5.00% |  |
Samsung Bioepis, ownership percentage |  |  | 49.90% |  |  | 49.90% |  |  |  |  |  | 15.00% | 15.00%
Payments to increase investment in Samsung Bioepis | $ 676.6 | ₩ 759.5 |  |  |  |  |  |  |  |  |  |  |
Investment in Samsung Bioepis |  |  | $ 620.2 | $ 580.2 |  | ₩ 673.8 |  |  |  | ₩ 670.8 |  | $ 45.0 | ₩ 49.5

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