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INCOME TAXES - Narrative (Details) - CAD ($)
$ in Millions
 12 Months Ended
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]        
Provincial corporate income tax rate   11.00%    
Increase in deferred income tax liability due to change in provincial corporate income tax rate $ 10      
Supplementary charge tax rate     10.00% 20.00%
Decrease in deferred corporate tax liability     $ 107  
Petroleum revenue tax rate     0.00% 35.00%
Recovery PRT rate for prior taxation years     50.00%  
Decrease in deferred PRT liability     $ 228  
Increase in deferred corporate income tax liability due to change in PRT rate     $ 114  
Unused tax losses        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deductible temporary differences for which no deferred tax asset is recognised 1,000      
Unused tax credits        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deductible temporary differences for which no deferred tax asset is recognised $ 750      
Changes in tax rate        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Provincial corporate income tax rate 12.00%