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Income Taxes - Significant Components of Deferred Income Tax Assets (Liabilities) (Details) - CAD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Writedown of deferred income tax assets $ (51) $ (18)
Net deferred tax liabilities (345) (454)
Gross carrying amount | Net operating loss carryforwards(1)    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Deferred tax liability (asset) 469 494
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) 140 122
Gross carrying amount | Property, plant and equipment    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Deferred tax liability (asset) (717) (828)
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) (107) (141)
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) 62 56
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) 22 42
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) 31 40
Gross carrying amount | Other deductible temporary differences    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Deferred tax liability (asset) 2 4
Accumulated impairment    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Deferred tax liability (asset) (98) (211)
Writedown of deferred income tax assets 247 243
Net deferred tax liabilities $ (345) $ (454)