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Summary of Significant Accounting Policies - Restricted Cash (Details) - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Accounting Policies [Abstract]        
Cash and cash equivalents $ 345 $ 563 $ 770  
Funds deposited by counterparties 32 33 37  
Restricted cash 8 17 279  
Cash and cash equivalents, funds deposited by counterparties and restricted cash shown in the statements of cash flows $ 385 $ 613 $ 1,086 $ 860