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FINANCIAL RISK MANAGEMENT (Financial assets and liabilities at fair value) (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value $ 178,673 $ 338,144
Financial liabilities, at fair value 12,981 6,001
Level 1    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value 177,085 335,840
Financial liabilities, at fair value 0 0
Level 2    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value 1,588 2,304
Financial liabilities, at fair value 12,981 6,001
Cash and cash equivalents    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value 140,455 236,335
Cash and cash equivalents | Level 1    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value 140,455 236,335
Cash and cash equivalents | Level 2    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value 0 0
Other investments    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value 36,630 99,505
Other investments | Level 1    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value 36,630 99,505
Other investments | Level 2    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value 0 0
Derivatives    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value 1,588 2,304
Derivatives | Level 1    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value 0 0
Derivatives | Level 2    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value 1,588 2,304
Derivatives    
Disclosure of fair value measurement of assets [line items]    
Financial liabilities, at fair value 12,981 6,001
Derivatives | Level 1    
Disclosure of fair value measurement of assets [line items]    
Financial liabilities, at fair value 0 0
Derivatives | Level 2    
Disclosure of fair value measurement of assets [line items]    
Financial liabilities, at fair value $ 12,981 $ 6,001