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Financial Instruments and Risk Management (Credit Risk - Aging of Trade Receivables) (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Gross [Member]    
Disclosure of financial instruments [Line Items]    
Trade receivables $ 993 $ 943
Gross [Member] | Not past due [Member]    
Disclosure of financial instruments [Line Items]    
Trade receivables 829 785
Gross [Member] | Past due up to 3 months [Member]    
Disclosure of financial instruments [Line Items]    
Trade receivables 114 125
Gross [Member] | Past due 3 to 12 months [Member]    
Disclosure of financial instruments [Line Items]    
Trade receivables 38 23
Gross [Member] | Past due over 12 months [Member]    
Disclosure of financial instruments [Line Items]    
Trade receivables 12 10
Accumulated impairment [Member]    
Disclosure of financial instruments [Line Items]    
Trade receivables (3) (11)
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 (1) (6)
Accumulated impairment [Member] | Past due over 12 months [Member]    
Disclosure of financial instruments [Line Items]    
Trade receivables $ (2) $ (5)