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Summary of Significant Accounting Policies - Cash and Cash Equivalents, Funds Deposited by Counterparties and Restricted Cash (Details) - USD ($)
$ in Millions
Sep. 30, 2021
Dec. 31, 2020
Sep. 30, 2020
Dec. 31, 2019
Accounting Policies [Abstract]        
Cash and cash equivalents $ 259 $ 3,905    
Funds deposited by counterparties 1,748 19    
Restricted cash 14 6    
Cash and cash equivalents, funds deposited by counterparties and restricted cash shown in the statement of cash flows $ 2,021 $ 3,930 $ 718 $ 385