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Summary of Significant Accounting Policies - Restricted Cash (Details) - USD ($)
$ in Millions
Mar. 31, 2021
Dec. 31, 2020
Mar. 31, 2020
Dec. 31, 2019
Accounting Policies [Abstract]        
Cash and cash equivalents $ 501 $ 3,905    
Funds deposited by counterparties 55 19    
Restricted cash 18 6    
Cash and cash equivalents, funds deposited by counterparties and restricted cash shown in the statement of cash flows $ 574 $ 3,930 $ 818 $ 385