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Summary of Significant Accounting Policies - Various Policies (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Foreign Currency Transactions/Translation |  |  |
Gain on revaluation of foreign currency transactions and monetary assets and liabilities | $ 16 | $ 1 | $ 1
Cash and Cash Equivalents and Restricted Cash |  |  |
Outstanding checks included in accounts payable that were reclassified from cash and cash equivalents | $ 27 | $ 39 |

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