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Income Taxes - Summary of Major Temporary Differences (Details) - CAD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax liabilities $ 13,194 $ 13,558    
Deferred income tax assets (2,655) (2,107)    
Net deferred income tax liability 10,539 11,451 $ 10,975 $ 9,073
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 12,074 12,885    
Lease assets        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax liabilities 412 0    
Unrealized risk management activities        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax liabilities 27 33    
PRT deduction for corporate income tax        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax liabilities 0 1    
Investments        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax liabilities 36 46    
Investment in North West Redwater Partnership        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax liabilities 593 414    
Asset retirement obligations        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax assets (1,488) (1,142)    
Lease liabilities        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax assets (416)      
Share-based compensation        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax assets (16) (5)    
Loss carryforwards        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax assets (685) (855)    
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 (49) (104)    
Deferred PRT        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax assets (1) (1)    
Other        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax liabilities $ 52 $ 179