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Derivative and Financial Instruments - FX Balance Sheet (Details) - USD ($)
$ in Millions
3 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Derivatives, Fair Value [Line Items]    
Description of Foreign Currency Derivative Instruments Not Designated as Hedging Instruments Activities In December 2017, we entered into foreign exchange zero cost collars buying the right to sell $1.25 billion CAD at $0.707 CAD and selling the right to buy $1.25 billion CAD at $0.842 CAD against the U.S. dollar.  
Prepaid Expenses and Other Current Assets [Member]    
Derivatives, Fair Value [Line Items]    
Foreign currency exchange derivative assets $ 18 $ 1
Other Assets [Member]    
Derivatives, Fair Value [Line Items]    
Foreign currency exchange derivative assets 0 6
Other Accruals [Member]    
Derivatives, Fair Value [Line Items]    
Foreign currency exchange derivative liabilities 7 0
Other liabilities and deferred credits [Member]    
Derivatives, Fair Value [Line Items]    
Foreign currency exchange derivative liabilities $ 0 $ 15