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FINANCIAL RISK MANAGEMENT (Financial assets and liabilities at fair value) (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value $ 338,144 $ 183,459
Financial liabilities, at fair value 6,001 287
Level 1    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value 335,840 178,131
Financial liabilities, at fair value 0 0
Level 2    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value 2,304 5,328
Financial liabilities, at fair value 6,001 287
Cash and cash equivalents    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value 236,335 100,026
Cash and cash equivalents | Level 1    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value 236,335 100,026
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 99,505 83,117
Other investments | Level 1    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value 99,505 78,105
Other investments | Level 2    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value 0 5,012
Derivatives    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value 2,304 316
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 2,304 316
Derivatives    
Disclosure of fair value measurement of assets [line items]    
Financial liabilities, at fair value 6,001 287
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 $ 6,001 $ 287