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CAPITAL AND FINANCIAL RISK MANAGEMENT - Market Risk management (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Interest Rate Risk    
CAPITAL AND FINANCIAL RISK MANAGEMENT    
Change in rate (as a percentage) 1.00%  
Impact on income before income and mining taxes and equity assumptions of sensitivity analysis $ 0 $ 2,600
Foreign Currency Risk    
CAPITAL AND FINANCIAL RISK MANAGEMENT    
Change in rate (as a percentage) 10.00%  
10.0% strengthening in U.S. Dollar | Canadian dollar    
CAPITAL AND FINANCIAL RISK MANAGEMENT    
Impact on income before income and mining taxes and equity assumptions of sensitivity analysis $ 8,435  
10.0% strengthening in U.S. Dollar | Euro    
CAPITAL AND FINANCIAL RISK MANAGEMENT    
Impact on income before income and mining taxes and equity assumptions of sensitivity analysis 2,477  
10.0% strengthening in U.S. Dollar | Mexican peso    
CAPITAL AND FINANCIAL RISK MANAGEMENT    
Impact on income before income and mining taxes and equity assumptions of sensitivity analysis (6,710)  
10.0% weakening in U.S. Dollar | Canadian dollar    
CAPITAL AND FINANCIAL RISK MANAGEMENT    
Impact on income before income and mining taxes and equity assumptions of sensitivity analysis (8,435)  
10.0% weakening in U.S. Dollar | Euro    
CAPITAL AND FINANCIAL RISK MANAGEMENT    
Impact on income before income and mining taxes and equity assumptions of sensitivity analysis (2,477)  
10.0% weakening in U.S. Dollar | Mexican peso    
CAPITAL AND FINANCIAL RISK MANAGEMENT    
Impact on income before income and mining taxes and equity assumptions of sensitivity analysis $ 6,710