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Income Taxes - Summary of Major Temporary Differences (Details) - CAD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax liabilities $ 13,091 $ 13,194    
Deferred income tax assets (2,947) (2,655)    
Net deferred income tax liability 10,144 10,539 $ 11,451 $ 10,975
Property, plant and equipment and exploration and evaluation assets        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax liabilities 11,922 12,074    
Lease assets        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax liabilities 380 412    
Unrealized risk management activities        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax liabilities 0 27    
Investments        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax liabilities 14 36    
Investment in North West Redwater Partnership        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax liabilities 767 593    
Other        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax liabilities 8 52    
Asset retirement obligations        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax assets (1,495) (1,488)    
Lease liabilities        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax assets (388) (416)    
Share-based compensation        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax assets (12) (16)    
Loss carryforwards        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax assets (1,032) (685)    
Unrealized foreign exchange loss (gain) on long-term debt        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax assets (20) (49)    
Deferred PRT        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax assets $ 0 $ (1)