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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