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Basis of Presentation (Schedule of Cash Activity) (Details) - USD ($)
$ in Millions
Sep. 30, 2021
Dec. 31, 2020
Sep. 30, 2020
Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]        
Cash and cash equivalents $ 886 $ 692 $ 1,564  
Program cash 55 72 94  
Restricted cash 4   2  
Total cash and cash equivalents, program and restricted cash $ 945 $ 765 $ 1,660 $ 900