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Income Taxes - Significant components of deferred tax assets and liabilities (Details) - CAD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |
Net deferred tax asset, recognized in earnings | $ (18,694) | $ (52,330) |
Deferred tax assets | 936,678 | 956,376 |
Deferred tax liabilities | 0 | 0 |
Net deferred tax asset | 936,678 | 956,376 | $ 1,006,012
Property, plant and equipment [member] |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |
Net deferred tax asset, recognized in earnings | (38,389) | 74,039 |
Net deferred tax assets | 280,798 | 319,185 |
Provision for reclamation [member] |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |
Net deferred tax asset, recognized in earnings | 28,628 | 2,325 |
Net deferred tax assets | 222,142 | 193,514 |
Inventories [Member] |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |
Net deferred tax asset, recognized in earnings | 4,071 | (2,163) |
Net deferred tax assets | 4,071 | 0 |
Net deferred tax liabilities | 0 | 301 |
Foreign exploration and development [member] |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |
Net deferred tax asset, recognized in earnings | 2 | (14) |
Net deferred tax assets | 5,269 | 5,267 |
Income tax losses (gains) [member] |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |
Net deferred tax asset, recognized in earnings | (7,629) | (108,839) |
Net deferred tax assets | 382,712 | 390,341 |
Defined benefit plan actuarial losses [member] |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |
Net deferred tax asset, recognized in earnings | 0 | 0 |
Net deferred tax assets | 9,410 | 7,947 |
Long-term investments and other, deferred tax assets [member] |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |
Net deferred tax asset, recognized in earnings | (5,678) | (17,377) |
Net deferred tax assets | 32,276 | 40,423 |
Deferred tax assets [member] |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |
Net deferred tax asset, recognized in earnings | (18,995) | (52,029) |
Net deferred tax assets | 936,678 | 956,677 |
Inventories [member] |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |
Net deferred tax asset, recognized in earnings | (301) | 301 |
Deferred tax liabilities [Member] |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |
Net deferred tax asset, recognized in earnings | (301) | 301 |
Net deferred tax liabilities | $ 0 | $ 301 |

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