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Financial instruments - related risk management (Details) - CAD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of detailed information about financial instruments [line items]      
Financial assets $ 1,425,387 $ 1,662,233  
Financial liabilities (1,203,910) (1,781,812)  
Gain (loss) on derivatives 32,269 (81,081)  
Finance income 29,760 22,071  
Disclosure of credit risk exposure [abstract]      
Cash and cash equivalents 1,062,431 711,528 $ 591,620
Short-term investments 0 391,025  
Advances receivable from JV Inkai LLP 0 124,533  
Derivative assets $ 10,504 3,881  
USD currency risk [member]      
Disclosure of detailed information about financial instruments [line items]      
Percentage increase or decrease in CAD$ 5.00%    
USD currency risk [member] | Cash and cash equivalents [member]      
Disclosure of detailed information about financial instruments [line items]      
Financial assets $ 120,675    
Gain (loss), foreign exchange 6,034    
USD currency risk [member] | Accounts receivable [Member]      
Disclosure of detailed information about financial instruments [line items]      
Financial assets 280,877    
Gain (loss), foreign exchange 14,044    
USD currency risk [member] | Net foreign currency derivatives [Member]      
Disclosure of detailed information about financial instruments [line items]      
Financial liabilities (4,333)    
Gain (loss), foreign exchange (30,851)    
Credit risk [member]      
Disclosure of credit risk exposure [abstract]      
Cash and cash equivalents 1,062,431 711,528  
Short-term investments 0 391,025  
Accounts receivable 323,430 398,639  
Advances receivable from JV Inkai LLP 0 124,533  
Derivative assets $ 10,504 $ 3,881