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Summary of Significant Accounting Policies - Restricted Cash (Details) - USD ($)
$ in Millions
Jun. 30, 2020
Dec. 31, 2019
Jun. 30, 2019
Dec. 31, 2018
Accounting Policies [Abstract]        
Cash and cash equivalents $ 418 $ 345    
Funds deposited by counterparties 36 32    
Restricted cash 8 8    
Cash and cash equivalents, funds deposited by counterparties and restricted cash shown in the statement of cash flows $ 462 $ 385 $ 336 $ 613