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FINANCIAL INSTRUMENTS - Carrying amounts and reconciliation of derivative financial instruments (Details) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of detailed information about hedging instruments [line items]      
Derivative financial asset (liability), net $ 101 $ 489 $ 854
Current portion of other long-term (liabilities) assets (103) 222  
Other long-term assets 204 267  
Cash flow hedges      
Disclosure of detailed information about hedging instruments [line items]      
Gain (loss) on hedge ineffectiveness 5 7 $ 5
Forward contracts      
Disclosure of detailed information about hedging instruments [line items]      
Derivatives held for trading, asset (liability) (38) 10  
Forward contracts | Cash flow hedges      
Disclosure of detailed information about hedging instruments [line items]      
Cash flow hedges, asset (liability) (71) 16  
AECO swaps      
Disclosure of detailed information about hedging instruments [line items]      
Derivatives held for trading, asset (liability) 0 (6)  
Cross currency swaps | Cash flow hedges      
Disclosure of detailed information about hedging instruments [line items]      
Cash flow hedges, asset (liability) $ 210 $ 469