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INCOME TAXES - Narrative (Details) - CAD ($)
$ in Millions
 6 Months Ended 12 Months Ended
Dec. 31, 2015
Jun. 30, 2015
Dec. 31, 2018
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]              
Supplementary charge tax rate         10.00% 20.00% 32.00%
Decrease in deferred corporate tax liability         $ 107    
Petroleum revenue tax rate         0.00% 35.00% 50.00%
Recovery PRT rate for prior taxation years         50.00% 50.00%  
Decrease in deferred PRT liability         $ 228 $ 11  
Increase in deferred corporate income tax liability due to change in PRT rate         $ 114    
Provincial corporate income tax rate 12.00% 10.00%   11.00%      
Increase in deferred income tax liability due to change in provincial corporate income tax rate       $ 10   579  
Decrease in deferred income tax liability due to change in PRT and supplementary charge on oil and gas profits           $ (217)  
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       $ 650      
Changes in tax rate              
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]              
Provincial corporate income tax rate     12.00%