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Income Taxes - Significant Components of Deferred Income Tax Assets (Liabilities) (Details) - CAD ($) $ in Millions | Dec. 31, 2019 | Dec. 31, 2018
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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)

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- Definition The amounts of income taxes recoverable in future periods in respect of: (a) deductible temporary differences; (b) the carryforward of unused tax losses; and (c) the carryforward of unused tax credits. [Refer: Temporary differences [member]; Unused tax credits [member]; Unused tax losses [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 12 -IssueDate 2018-01-01 -Paragraph 81 -Subparagraph g -Clause i -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=12&date=2018-03-01&anchor=para_81_g_i&doctype=Standard -URIDate 2018-03-16 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 1 -IssueDate 2018-01-01 -Paragraph 54 -Subparagraph o -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2018-03-01&anchor=para_54_o&doctype=Standard -URIDate 2018-03-16 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 1 -IssueDate 2018-01-01 -Paragraph 56 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2018-03-01&anchor=para_56&doctype=Standard -URIDate 2018-03-16 + Details
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- Definition The amount of deferred tax liabilities or assets. [Refer: Deferred tax liabilities; Deferred tax assets] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 12 -IssueDate 2018-01-01 -Paragraph 81 -Subparagraph g -Clause i -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=12&date=2018-03-01&anchor=para_81_g_i&doctype=Standard -URIDate 2018-03-16 + Details
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