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Financial instruments - Measurement of fair values hierarchy levels (Details) - CAD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Disclosure of detailed information about financial instruments [line items]    
Derivative assets $ 40,804 $ 10,612
Investments in equity securities 21,417 14,807
Derivative liabilities (23,414) (58,885)
Long-term debt (1,494,471) (1,493,327)
Net financial instruments (1,455,664) (1,526,793)
Foreign currency contracts [Member]    
Disclosure of detailed information about financial instruments [line items]    
Derivative assets 39,984 4,065
Derivative liabilities (5,624) (29,231)
Interest rate contracts [member]    
Disclosure of detailed information about financial instruments [line items]    
Derivative assets 820 6,547
Derivative liabilities (970)  
Uranium contracts [Member]    
Disclosure of detailed information about financial instruments [line items]    
Derivative liabilities (16,820) (29,654)
Share purchase options [Member]    
Disclosure of detailed information about financial instruments [line items]    
Derivative liabilities   (29,654)
Level 1 of fair value hierarchy [member]    
Disclosure of detailed information about financial instruments [line items]    
Investments in equity securities 21,417 14,807
Net financial instruments 21,417 14,807
Level 2 of fair value hierarchy [member]    
Disclosure of detailed information about financial instruments [line items]    
Long-term debt (1,652,230) (1,721,805)
Net financial instruments (1,634,840) (1,770,078)
Level 2 of fair value hierarchy [member] | Foreign currency contracts [Member]    
Disclosure of detailed information about financial instruments [line items]    
Derivative assets 39,984 4,065
Derivative liabilities (5,624) (29,231)
Level 2 of fair value hierarchy [member] | Interest rate contracts [member]    
Disclosure of detailed information about financial instruments [line items]    
Derivative assets 820 6,547
Derivative liabilities (970)  
Level 2 of fair value hierarchy [member] | Uranium contracts [Member]    
Disclosure of detailed information about financial instruments [line items]    
Derivative liabilities (16,820)  
Level 2 of fair value hierarchy [member] | Share purchase options [Member]    
Disclosure of detailed information about financial instruments [line items]    
Derivative liabilities   (29,654)
Fair value [member]    
Disclosure of detailed information about financial instruments [line items]    
Investments in equity securities 21,417 14,807
Long-term debt (1,652,230) (1,721,805)
Net financial instruments (1,613,423) (1,755,271)
Fair value [member] | Foreign currency contracts [Member]    
Disclosure of detailed information about financial instruments [line items]    
Derivative assets 39,984 4,065
Derivative liabilities (5,624) (29,231)
Fair value [member] | Interest rate contracts [member]    
Disclosure of detailed information about financial instruments [line items]    
Derivative assets 820 6,547
Derivative liabilities (970)  
Fair value [member] | Uranium contracts [Member]    
Disclosure of detailed information about financial instruments [line items]    
Derivative liabilities $ (16,820)  
Fair value [member] | Share purchase options [Member]    
Disclosure of detailed information about financial instruments [line items]    
Derivative liabilities   $ (29,654)