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Cash, Cash Equivalents and Restricted Cash (Details) - USD ($)
$ in Millions
Jun. 30, 2019
Dec. 31, 2018
Jun. 30, 2018
Dec. 31, 2017
Restricted Cash [Abstract]        
Cash and cash equivalents $ 342 $ 607    
Restricted cash included in prepaid expenses and other current assets 48 45    
Restricted cash included in other non-current assets 7 7    
Total restricted cash 55 52    
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents $ 397 $ 659 $ 307 $ 435