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Income Taxes - Deferred Income Tax Asset (Details) - CAD CAD in Millions | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |  |  |  |  |
Deferred income tax assets | CAD 100.0 | CAD 63.0 |  |  | CAD 100.0 | CAD 63.0 |
Capital losses on foreign exchange |  |  |  |  | 556.0 | 926.0 |
Deferred income tax liabilities | 11,533.0 | 11,243.0 |  |  | 11,533.0 | 11,243.0 |
Deferred income tax liability (asset) | 11,433.0 | 11,180.0 |  |  | CAD 11,433.0 | CAD 11,180.0 | CAD 9,919.0
Tax rate |  |  |  |  | 27.01% | 27.00% |
Tax loss carryforwards |  |  |  |  |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |  |  |  |  |
Deferred income tax assets | 75.0 | 125.0 |  |  | CAD 75.0 | CAD 125.0 |
Earnings retained in our investments in foreign subsidiaries |  |  |  |  |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |  |  |  |  |
Deferred income tax liabilities | CAD 9,600.0 | CAD 9,900.0 |  |  | CAD 9,600.0 | CAD 9,900.0 |
Change in corporate income tax rate | Government Of Quebec |  |  |  |  |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |  |  |  |  |
Tax rate |  | 11.90% |  |  |  |  |
Enacted tax rate |  | 11.50% |  |  |  |  |
Term in which the enacted tax rate will be in effect |  | 4 years |  |  |  |  |
Deferred income tax expense (recovery) |  | CAD 10.0 |  |  |  |  |
Change in corporate income tax rate | Government of U.K. |  |  |  |  |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |  |  |  |  |
Tax rate |  |  | 50.00% |  |  |  |
Enacted tax rate |  |  | 40.00% |  |  |  |
Deferred income tax expense (recovery) |  |  | CAD 180.0 |  |  |  |
United States Government |  |  |  |  |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |  |  |  |  |
Tax rate |  |  |  |  | 35.00% |  |
Deferred income tax expense (recovery) |  |  |  |  | CAD (124.0) |  |
United States Government | Scenario, Forecast [Member] |  |  |  |  |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |  |  |  |  |
Tax rate |  |  |  | 21.00% |  |  |
Government of British Columbia |  |  |  |  |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |  |  |  |  |
Tax rate | 11.00% |  |  |  |  |  |
Enacted tax rate | 12.00% |  |  |  |  |  |
Deferred income tax expense (recovery) | CAD 18.0 |  |  |  |  |  |

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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 2017-01-01 -Paragraph 81 -Subparagraph g -Clause i -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=12&date=2017-03-01&anchor=para_81_g_i&doctype=Standard -URIDate 2017-03-09 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 1 -IssueDate 2017-01-01 -Paragraph 54 -Subparagraph o -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_54_o&doctype=Standard -URIDate 2017-03-09 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 1 -IssueDate 2017-01-01 -Paragraph 56 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_56&doctype=Standard -URIDate 2017-03-09 + Details
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