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Summary of Significant Accounting Policies Detail of Cash and Cash Equivalents, Program and Restricted Cash (Details) - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Cash and Cash Equivalents, Program and Restricted Cash [Abstract]        
Cash and Cash Equivalents, at Carrying Value $ 686 $ 615    
Program Cash 211 115    
Restricted Cash and Cash Equivalents 3 5    
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents $ 900 $ 735 $ 901 $ 720