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DERIVATIVE FINANCIAL INSTRUMENTS - Currency Risk Management (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Foreign Exchange Zero Cost Collars    
Disclosure of detailed information about financial instruments [line items]    
Net premium payable $ 0  
U S Dollar Call Options    
Disclosure of detailed information about financial instruments [line items]    
Notional amount 0 $ 0
2018 | Foreign Exchange Zero Cost Collars    
Disclosure of detailed information about financial instruments [line items]    
Estimated expenditure that is being hedged 84,000  
2018 | Foreign Exchange Zero Cost Collars | Cash flow hedges    
Disclosure of detailed information about financial instruments [line items]    
Estimated expenditure that is being hedged $ 276,000