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Financial Instruments and Risk Management (Credit Risk - Aging of Trade Receivables) (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Gross [Member]    
Disclosure of financial instruments [Line Items]    
Trade receivables $ 943 $ 972
Gross [Member] | Not past due [Member]    
Disclosure of financial instruments [Line Items]    
Trade receivables 785 832
Gross [Member] | Past due up to 3 months [Member]    
Disclosure of financial instruments [Line Items]    
Trade receivables 125 91
Gross [Member] | Past due 3 to 12 months [Member]    
Disclosure of financial instruments [Line Items]    
Trade receivables 23 44
Gross [Member] | Past due over 12 months [Member]    
Disclosure of financial instruments [Line Items]    
Trade receivables 10 5
Accumulated impairment [Member]    
Disclosure of financial instruments [Line Items]    
Trade receivables (11) (6)
Accumulated impairment [Member] | Not past due [Member]    
Disclosure of financial instruments [Line Items]    
Trade receivables 0 0
Accumulated impairment [Member] | Past due up to 3 months [Member]    
Disclosure of financial instruments [Line Items]    
Trade receivables 0 0
Accumulated impairment [Member] | Past due 3 to 12 months [Member]    
Disclosure of financial instruments [Line Items]    
Trade receivables (6) (1)
Accumulated impairment [Member] | Past due over 12 months [Member]    
Disclosure of financial instruments [Line Items]    
Trade receivables $ (5) $ (5)