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Risk Management - Impact of Fluctuating Commodity Prices and Interest Rates on Company'S Open Risk Management Positions (Detail) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Commodity Price Risk [Member] | Crude Oil Contracts [Member]    
Disclosure of Nature and Extent of Risks Arising from Financial Instruments [Line Items]    
Description of exposure to risk Crude Oil Commodity Price Crude Oil Commodity Price
Sensitivity Range ± US$5.00 per bbl Applied to WTI and Condensate Hedges ± US$5.00 per bbl Applied to WTI and Condensate Hedges
Increase $ (44) $ 3
Decrease $ 44 $ (3)
Commodity Price Risk [Member] | Crude oil differential contracts [Member]    
Disclosure of Nature and Extent of Risks Arising from Financial Instruments [Line Items]    
Description of exposure to risk Crude Oil Differential Price Crude Oil Differential Price
Sensitivity Range ± US$2.50 per bbl Applied to Differential Hedges Tied to Production ± US$2.50 per bbl Applied to Differential Hedges Tied to Production
Increase $ (2) $ 5
Decrease 2 (5)
Currency risk [Member]    
Disclosure of Nature and Extent of Risks Arising from Financial Instruments [Line Items]    
$0.05 Increase in the Canadian per U.S. Dollar Foreign Exchange Rate 300 250
$0.05 Decrease in the Canadian per U.S. Dollar Foreign Exchange Rate $ (300) $ (250)