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Impairment Charges and Reversals - Summary of Increase (Decrease) to Recoverable Amount (Detail)
$ in Millions
 12 Months Ended
Dec. 31, 2020
CAD ($)
Clearwater [Member] | One Percent Increase in the Discount Rate [Member]  
Disclosure of estimation uncertainty with significant risk of causing material adjustment [line items]  
Increase (decrease) to recoverable amount $ (5)
Clearwater [Member] | One Percent Decrease in the Discount Rate [Member]  
Disclosure of estimation uncertainty with significant risk of causing material adjustment [line items]  
Increase (decrease) to recoverable amount 6
Clearwater [Member] | Five Percent Increase in the Forward Price Estimates [Member]  
Disclosure of estimation uncertainty with significant risk of causing material adjustment [line items]  
Increase (decrease) to recoverable amount 52
Clearwater [Member] | Five Percent Decrease in the Forward Price Estimates [Member]  
Disclosure of estimation uncertainty with significant risk of causing material adjustment [line items]  
Increase (decrease) to recoverable amount (97)
Elmworth-Wapiti [Member] | One Percent Increase in the Discount Rate [Member]  
Disclosure of estimation uncertainty with significant risk of causing material adjustment [line items]  
Increase (decrease) to recoverable amount (7)
Elmworth-Wapiti [Member] | One Percent Decrease in the Discount Rate [Member]  
Disclosure of estimation uncertainty with significant risk of causing material adjustment [line items]  
Increase (decrease) to recoverable amount 8
Elmworth-Wapiti [Member] | Five Percent Increase in the Forward Price Estimates [Member]  
Disclosure of estimation uncertainty with significant risk of causing material adjustment [line items]  
Increase (decrease) to recoverable amount 54
Elmworth-Wapiti [Member] | Five Percent Decrease in the Forward Price Estimates [Member]  
Disclosure of estimation uncertainty with significant risk of causing material adjustment [line items]  
Increase (decrease) to recoverable amount (96)
Kaybob-Edson [Member] | One Percent Increase in the Discount Rate [Member]  
Disclosure of estimation uncertainty with significant risk of causing material adjustment [line items]  
Increase (decrease) to recoverable amount (13)
Kaybob-Edson [Member] | One Percent Decrease in the Discount Rate [Member]  
Disclosure of estimation uncertainty with significant risk of causing material adjustment [line items]  
Increase (decrease) to recoverable amount 14
Kaybob-Edson [Member] | Five Percent Increase in the Forward Price Estimates [Member]  
Disclosure of estimation uncertainty with significant risk of causing material adjustment [line items]  
Increase (decrease) to recoverable amount 54
Kaybob-Edson [Member] | Five Percent Decrease in the Forward Price Estimates [Member]  
Disclosure of estimation uncertainty with significant risk of causing material adjustment [line items]  
Increase (decrease) to recoverable amount (106)
Borger [Member] | One Percent Increase in the Discount Rate [Member]  
Disclosure of estimation uncertainty with significant risk of causing material adjustment [line items]  
Increase (decrease) to recoverable amount (71)
Borger [Member] | One Percent Decrease in the Discount Rate [Member]  
Disclosure of estimation uncertainty with significant risk of causing material adjustment [line items]  
Increase (decrease) to recoverable amount 81
Borger [Member] | Five Percent Increase in the Forward Price Estimates [Member]  
Disclosure of estimation uncertainty with significant risk of causing material adjustment [line items]  
Increase (decrease) to recoverable amount 263
Borger [Member] | Five Percent Decrease in the Forward Price Estimates [Member]  
Disclosure of estimation uncertainty with significant risk of causing material adjustment [line items]  
Increase (decrease) to recoverable amount $ (264)