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Summary of Significant Accounting Policies - Restricted Cash (Details) - USD ($)
$ in Millions
Sep. 30, 2018
Dec. 31, 2017
Sep. 30, 2017
Dec. 31, 2016
Accounting Policies [Abstract]        
Cash and cash equivalents $ 1,359 $ 767 $ 1,022 $ 591
Funds deposited by counterparties 30 37 31 2
Restricted cash 28 279 318 267
Cash and cash equivalents, funds deposited by counterparties and restricted cash shown in the statement of cash flows $ 1,417 $ 1,083 $ 1,371 $ 860