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Income Taxes - Significant Components of Deferred Income Tax Assets (Liabilities) (Details) - CAD ($)
$ in Millions
Dec. 31, 2021
Dec. 31, 2020
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Unrecognized deferred income tax assets $ (64) $ (51)
Net deferred tax liabilities (290) (345)
Gross carrying amount | Net operating loss carryforwards    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Deferred tax liability (asset) 530 469
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) 183 140
Gross carrying amount | Property, plant and equipment    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Deferred tax liability (asset) (651) (717)
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) (53) (107)
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) 53 62
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) 17 22
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) 16 31
Gross carrying amount | Other deductible temporary differences    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Deferred tax liability (asset) (5) 2
Accumulated impairment    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Deferred tax liability (asset) 90 (98)
Unrecognized deferred income tax assets $ (380) $ (247)