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Accounting Policies - Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($)
$ in Millions
Sep. 30, 2019
Dec. 31, 2018
Sep. 30, 2018
Dec. 31, 2017
Accounting Policies [Abstract]        
Cash and Cash Equivalents $ 868 $ 801 $ 896  
Restricted Cash 58   66  
Total Cash, Cash Equivalents and Restricted Cash $ 926 $ 873 $ 962 $ 1,110