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Summary of Significant Accounting Policies - Restricted Cash (Details) - USD ($)
$ in Millions
Jun. 30, 2019
Dec. 31, 2018
Jun. 30, 2018
Dec. 31, 2017
Accounting Policies [Abstract]        
Cash and cash equivalents $ 294 $ 563    
Funds deposited by counterparties 31 33    
Restricted cash 11 17    
Cash and cash equivalents, funds deposited by counterparties and restricted cash shown in the statement of cash flows $ 336 $ 613 $ 877 $ 1,086