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Cash, Cash Equivalents and Restricted Cash (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Cash and Cash Equivalents [Line Items]        
Cash and cash equivalents $ 1,041 $ 951 $ 1,196 $ 1,085
Cash, cash equivalents and restricted cash as presented in our Consolidated Condensed Statements of Cash Flows 1,124 1,084 1,293 1,240
Foreign currency translation adjustment (3) (1)    
Prepaid Expenses and Other Current Assets        
Cash and Cash Equivalents [Line Items]        
Restricted cash 49 45 48 45
Other Noncurrent Assets        
Cash and Cash Equivalents [Line Items]        
Restricted cash $ 34 $ 88 $ 49 $ 110