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INCOME TAXES - Summary of Major Temporary Differences (Details) - CAD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax liabilities $ 12,859 $ 10,670    
Deferred income tax assets (1,884) (1,597)    
Net deferred income tax liability 10,975 9,073 $ 9,344 $ 8,970
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 12,484 10,259    
Unrealized risk management activities        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax liabilities 20 62    
PRT deduction for corporate income tax        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax liabilities 7 29    
Investments        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax liabilities 96 98    
Investment in North West Redwater Partnership        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax liabilities 252 222    
Asset retirement obligations        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax assets (1,264) (983)    
Loss carryforwards        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax assets (523) (390)    
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 (29) (149)    
Deferred PRT        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax assets (18) (73)    
Other        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax assets $ (50) $ (2)