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Cash and Cash Equivalents (Summary of Cash and Cash Equivalents) (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 2017
Dec. 31, 2016
Cash and Cash Equivalents [Abstract]        
Cash $ 20 $ 261    
Marketable securities 17 605    
Other cash equivalents   50    
Total $ 37 $ 916 $ 1,111 $ 1,423