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Income Taxes - Significant Components of Deferred Income Tax Assets (Liabilities) (Details) - CAD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Writedown of deferred income tax assets $ (18) $ (28)
Net deferred tax liabilities (454) (473)
Gross carrying amount | Net operating loss carryforwards    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Deferred tax liability (asset) 494 547
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) 122 113
Gross carrying amount | Property, plant and equipment    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Deferred tax liability (asset) (828) (896)
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) (141) (145)
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) 56 68
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) 42 48
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) 40 35
Gross carrying amount | Deferred coal revenues    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Deferred tax liability (asset) 0 23
Gross carrying amount | Other deductible temporary differences    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Deferred tax liability (asset) 4 0
Accumulated impairment    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Deferred tax liability (asset) (211) (207)
Writedown of deferred income tax assets 243 266
Net deferred tax liabilities $ (454) $ (473)