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Financial Instruments and Risk Management (Credit Risk - Movement in the Allowance of Doubtful Accounts during the Year) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Notes to Consolidated Financial Statements [Abstract]    
Balance at start of period $ 6 $ 11
Additional allowance 5 1
Write offs (1) (3)
Reversals 0 (2)
Change due to translation differences 1 (1)
Balance at end of period $ 11 $ 6