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Income Taxes - Deferred Income Tax Asset (Details) - CAD ($) $ in Millions | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |  |
Deferred income tax assets | $ 100.0 | $ 128.0 | $ 100.0 |
Capital losses on foreign exchange |  | 1,134.0 | 556.0 |
Deferred income tax liabilities | 11,533.0 | 12,045.0 | 11,533.0 |
Deferred income tax liability (asset) | 11,433.0 | $ 11,917.0 | $ 11,433.0 | $ 11,180.0
Tax rate |  | 27.04% | 27.01% |
Tax loss carryforwards |  |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |  |
Deferred income tax assets | 75.0 | $ 153.0 | $ 75.0 |
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 | $ 9,600.0 | $ 9,700.0 | $ 9,600.0 |
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) |  | $ (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) | $ 18.0 |  |  |

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