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Decommissioning Liabilities - Additional Information (Detail) - CAD ($)
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Disclosure Of Other Provisions [Line Items]    
Estimated future cash flows required to settle the obligation $ 4,953,000,000 $ 5,173,000,000
Credit-adjusted risk-free rate 5.00% 4.90%
Inflation Rate 2.00% 2.00%
Bottom of Range [Member]    
Disclosure Of Other Provisions [Line Items]    
Settlement Of Decommissioning Liabilities $ 40,000,000  
Top of Range [Member]    
Disclosure Of Other Provisions [Line Items]    
Settlement Of Decommissioning Liabilities $ 45,000,000