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Financial instruments - related risk management (Details) - CAD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of detailed information about financial instruments [line items]      
Financial assets $ 1,662,233 $ 1,109,485  
Financial liabilities (1,781,812) (1,815,869)  
Gain (loss) on derivatives (81,081) 56,250  
Finance income 22,071 5,265  
Disclosure of credit risk exposure [abstract]      
Cash and cash equivalents 711,528 591,620 $ 320,278
Short-term investments 391,025 0  
Advances receivable from JV Inkai LLP 124,533 58,820  
Derivative assets $ 3,881 40,804  
Currency risk [member]      
Disclosure of detailed information about financial instruments [line items]      
Percentage increase or decrease in CAD$ 5.00%    
Currency risk [member] | Cash and cash equivalents [member]      
Disclosure of detailed information about financial instruments [line items]      
Gain (loss), foreign exchange $ 5,273    
Currency risk [member] | Cash and cash equivalents [member] | USD [Member]      
Disclosure of detailed information about financial instruments [line items]      
Financial assets 105,463    
Currency risk [member] | Accounts receivable [Member]      
Disclosure of detailed information about financial instruments [line items]      
Gain (loss), foreign exchange 17,945    
Currency risk [member] | Accounts receivable [Member] | USD [Member]      
Disclosure of detailed information about financial instruments [line items]      
Financial assets 358,904    
Currency risk [member] | Long-term receivables, investments and other [Member]      
Disclosure of detailed information about financial instruments [line items]      
Gain (loss), foreign exchange 6,227    
Currency risk [member] | Long-term receivables, investments and other [Member] | USD [Member]      
Disclosure of detailed information about financial instruments [line items]      
Financial assets 124,533    
Currency risk [member] | Accounts Payable and Accrued Liabilities [Member]      
Disclosure of detailed information about financial instruments [line items]      
Gain (loss), foreign exchange (2,274)    
Currency risk [member] | Accounts Payable and Accrued Liabilities [Member] | USD [Member]      
Disclosure of detailed information about financial instruments [line items]      
Financial liabilities 45,474    
Currency risk [member] | Net foreign currency derivatives [Member]      
Disclosure of detailed information about financial instruments [line items]      
Gain (loss), foreign exchange (48,923)    
Currency risk [member] | Net foreign currency derivatives [Member] | USD [Member]      
Disclosure of detailed information about financial instruments [line items]      
Financial liabilities $ 52,665    
Interest rate risk [member]      
Disclosure of detailed information about financial instruments [line items]      
Percentage increase or decrease in the interest rate 1.00%    
Interest rate risk [member] | Interest rate contracts [member]      
Disclosure of detailed information about financial instruments [line items]      
Gain (loss) on derivatives $ (4,173)    
Interest rate risk [member] | Advances receivable from Inkai [Member]      
Disclosure of detailed information about financial instruments [line items]      
Finance income 1,179    
Credit risk [member]      
Disclosure of credit risk exposure [abstract]      
Cash and cash equivalents 711,528 591,620  
Short-term investments 391,025 0  
Accounts receivable 398,639 393,213  
Advances receivable from JV Inkai LLP 124,533 58,820  
Derivative assets $ 3,881 $ 40,804