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Income Taxes - Significant components of unrecognized deferred tax assets(Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
USD ($)
Dec. 31, 2017
CAD ($)
Dec. 31, 2016
CAD ($)
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Unrecognized deferred tax assets   $ 401,853 $ 435,972
Tax credits $ 4,073,000    
Range of years when minimum tax credit will be refunded 2018 and 2021    
Income tax losses [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Unrecognized deferred tax assets   259,770 284,338
Property, plant and equipment, deferred tax assets [Member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Unrecognized deferred tax assets   2,076 3,789
Provision for reclamation [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Unrecognized deferred tax assets   71,463 40,749
Long-term investments and other, deferred tax assets [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Unrecognized deferred tax assets   $ 68,544 $ 107,096