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Summary of Significant Accounting Policies - Restricted Cash (Details) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Accounting Policies [Abstract]        
Cash and cash equivalents $ 3,905 $ 345 $ 563  
Funds deposited by counterparties 19 32 33  
Restricted cash 6 8 17  
Cash and cash equivalents, funds deposited by counterparties and restricted cash shown in the statements of cash flows $ 3,930 $ 385 $ 613 $ 1,086