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Income Taxes - Significant components of deferred tax assets and liabilities (Details) - CAD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Net deferred tax asset, recognized in earnings $ (52,330) $ 147,091  
Deferred tax assets 956,376 1,006,012  
Deferred tax liabilities 0 0  
Net deferred tax asset 956,376 1,006,012 $ 848,704
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 74,039 119,132  
Net deferred tax assets 319,185 245,206  
Provision for reclamation [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Net deferred tax asset, recognized in earnings 2,325 (36,622)  
Net deferred tax assets 193,514 191,189  
Inventories [Member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Net deferred tax asset, recognized in earnings (2,163) 1,137  
Net deferred tax assets 0 2,163  
Net deferred tax liabilities 301 0  
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 (14) (14)  
Net deferred tax assets 5,267 5,281  
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 (108,839) 39,289  
Net deferred tax assets 390,341 499,180  
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 7,947 5,646  
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 (17,377) 24,169  
Net deferred tax assets 40,423 57,347  
Deferred tax assets [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Net deferred tax asset, recognized in earnings (52,029) 147,091  
Net deferred tax assets 956,677 1,006,012  
Inventories [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Net deferred tax asset, recognized in earnings 301 0  
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 0  
Net deferred tax liabilities $ 301 $ 0