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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% |  |  |  |

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