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Summary of Significant Accounting Policies - Restricted Cash (Details) - USD ($)
$ in Millions
Mar. 31, 2019
Dec. 31, 2018
Mar. 31, 2018
Dec. 31, 2017
Accounting Policies [Abstract]        
Cash and cash equivalents $ 859 $ 563 $ 514 $ 770
Funds deposited by counterparties 11 33 241 37
Restricted cash 15 17 261 279
Cash and cash equivalents, funds deposited by counterparties and restricted cash shown in the statement of cash flows $ 885 $ 613 $ 1,016 $ 1,086