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Exploration and evaluation (Narrative) (Details) - CAD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Statements Line Items      
Impairment $ 49,986 $ 7,344  
Impairment charge, net of income taxes 37,600 5,400  
Net book value 42,949 95,002 $ 102,182
O3 Mining Inc [Member]      
Statements Line Items      
Net book value 31,700 $ 34,000  
Investments on the properties by Farmee 2,000    
Total earnout amount 6,600    
Coulon Project [Member]      
Statements Line Items      
Estimated recoverable amount $ 10,000    
Exploration Properties [Member]      
Statements Line Items      
Net smelter return on other metals and minerals 2.00%    
Exploration Properties [Member] | O3 Mining Inc [Member]      
Statements Line Items      
Earn-in agreement description O3 Mining may earn a 100% interest in most of Osisko's exploration properties located in the James Bay area and Labrador Trough (excluding the Coulon copper-zinc project) upon completing expenditures of $26.0 million over an initial 7-year period; O3 Mining may earn a first 50% interest upon completing expenditures totaling $15.6 million over an initial 4-year period.    
Osisko Mining [Member] | Kan property [Member]      
Statements Line Items      
Earn-in agreement description Osisko Mining reached the minimal investment threshold on the Kan property. Therefore, a 100% interest in the Kan property was transferred to Osisko Mining (now held by O3 Mining Inc.)    
Net smelter return on other metals and minerals 2.00%    
Bottom of range [member] | Exploration Properties [Member]      
Statements Line Items      
Net smelter return on other metals and minerals 1.50%    
Bottom of range [member] | Osisko Mining [Member] | Kan property [Member]      
Statements Line Items      
Net smelter return on other metals and minerals 1.50%    
Top of range [member] | Exploration Properties [Member]      
Statements Line Items      
Net smelter return on other metals and minerals 3.50%    
Top of range [member] | Osisko Mining [Member] | Kan property [Member]      
Statements Line Items      
Net smelter return on other metals and minerals 3.50%