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Income Taxes - Significant Components of Deferred Income Tax Assets (Liabilities) (Details) - CAD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Writedown of deferred income tax assets $ (28) $ (24)
Net deferred tax liabilities (473) (525)
Gross carrying amount | Net operating loss carryforwards    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Deferred tax liability (asset) 547 541
Gross carrying amount | Future decommissioning and restoration costs    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Deferred tax liability (asset) 113 117
Gross carrying amount | Property, plant and equipment    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Deferred tax liability (asset) (896) (1,009)
Gross carrying amount | Risk management assets and liabilities, net    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Deferred tax liability (asset) (145) (160)
Gross carrying amount | Employee future benefits and compensation plans    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Deferred tax liability (asset) 68 74
Gross carrying amount | Interest deductible in future periods    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Deferred tax liability (asset) 48 50
Gross carrying amount | Foreign exchange differences on US-denominated debt    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Deferred tax liability (asset) 35 42
Gross carrying amount | Deferred coal revenues    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Deferred tax liability (asset) 23 16
Gross carrying amount | Other deductible temporary differences    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Deferred tax liability (asset) 0 22
Accumulated impairment    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Deferred tax liability (asset) (207) (307)
Writedown of deferred income tax assets 266 218
Net deferred tax liabilities $ (473) $ (525)