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INCOME TAXES - Summary of Major Temporary Differences (Details) - CAD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax liabilities $ 13,553 $ 12,859    
Deferred income tax assets (2,102) (1,884)    
Net deferred income tax liability 11,451 10,975 $ 9,073 $ 9,344
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 12,885 12,484    
Unrealized risk management activities        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax liabilities 33 20    
PRT deduction for corporate income tax        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax liabilities 1 7    
Investments        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax liabilities 46 96    
Investment in North West Redwater Partnership        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax liabilities 414 252    
Asset retirement obligations        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax assets (1,142) (1,264)    
Loss carryforwards        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax assets (855) (523)    
Unrealized foreign exchange (gain) loss on long-term debt        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax assets (104) (29)    
Deferred PRT        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax assets (1) (18)    
Other        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax liabilities 174 0    
Deferred income tax assets $ 0 $ (50)