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Income Taxes - Additional Information (Detail) - CAD ($)
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of income taxes [line items]      
Canadian Statutory Tax Rate 24.00% 26.50% 27.00%
Deferred tax (expense) recovery   $ 387,000,000 $ 78,000,000
Deferred income tax assets has been recognized $ 36,000,000    
Deferred tax liability has been recognized 0 0  
Amounts of tax pools available, including tax losses 9,657,000,000 8,761,000,000  
Net capital losses recognized as a deferred income tax asset 22,000,000 100,000,000  
Canadian Federal Non Capital Losses [Member]      
Disclosure of income taxes [line items]      
Amounts of tax pools available, including tax losses 1,682,000,000 696,000,000  
US Federal Net Operating Losses [Member]      
Disclosure of income taxes [line items]      
Amounts of tax pools available, including tax losses 1,084,000,000 188,000,000  
Canadian Net Capital Losses [Member]      
Disclosure of income taxes [line items]      
Amounts of tax pools available, including tax losses 85,000,000 188,000,000  
Net capital losses associated with unrealized foreign exchange losses $ 254,000,000 262,000,000  
Internal Restructuring [Member]      
Disclosure of income taxes [line items]      
Deferred tax (expense) recovery   $ 387,000,000  
Alberta [Member]      
Disclosure of income taxes [line items]      
Canadian Statutory Tax Rate 8.00% 8.00% 12.00%
Reduction in provincial corporate tax rate enacted period   4 years  
Deferred tax (expense) recovery   $ 671,000,000