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Summary of Significant Accounting Policies - Recently Adopted Accounting Pronouncements (Details) - USD ($) $ in Millions | 6 Months Ended |
--- | --- | ---
Jun. 30, 2018 | Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |
Cumulative effect from the adoption of new accounting standards |  | $ 190
Galapagos |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |
Unrealized gain of investment | $ 278 |
Accumulated Other Comprehensive Income (Loss) |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |
Cumulative effect from the adoption of new accounting standards |  | (293)
Accumulated Other Comprehensive Income (Loss) | Accounting Standards Update 2016-01 |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |
Cumulative effect from the adoption of new accounting standards |  | (293)
Retained Earnings |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |
Cumulative effect from the adoption of new accounting standards |  | 483
Retained Earnings | Accounting Standards Update 2016-01 |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |
Cumulative effect from the adoption of new accounting standards |  | $ 293

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