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Financial Instruments - Carrying Amounts and Reconciliation of Derivative Financial Instruments (Details) - CAD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of detailed information about hedging instruments [line items]      
Derivative financial asset (liability), net $ 178 $ 356 $ 101
Current portion of other long-term assets 8 92  
Current portion of other long-term liabilities (112) (17)  
Other long-term assets 282 281  
Cash flow hedges      
Disclosure of detailed information about hedging instruments [line items]      
Gain (loss) on hedge ineffectiveness 3 2 $ 5
Forward contracts      
Disclosure of detailed information about hedging instruments [line items]      
Derivatives held for trading, asset (liability) (10) 8  
Forward contracts | Cash flow hedges      
Disclosure of detailed information about hedging instruments [line items]      
Cash flow hedges, asset (liability) (91) 70  
AECO basis swap      
Disclosure of detailed information about hedging instruments [line items]      
Derivatives held for trading, asset (liability) (8) 1  
AECO fixed price swaps      
Disclosure of detailed information about hedging instruments [line items]      
Derivatives held for trading, asset (liability) (3) 3  
WCS differential swaps      
Disclosure of detailed information about hedging instruments [line items]      
Derivatives held for trading, asset (liability) 0 (17)  
Cross currency swaps | Cash flow hedges      
Disclosure of detailed information about hedging instruments [line items]      
Cash flow hedges, asset (liability) $ 290 $ 291