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DERIVATIVE INSTRUMENTS (Derivative Instruments in Financial Position) (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
General Discussion of Derivative Instruments and Hedging Activities [Abstract]    
Short-term derivative instruments - asset $ 17,958 $ 21,352
Long-term derivative instruments - asset 0 0
Short-term derivative instruments - liability 25,921 20,401
Long-term derivative instruments - liability $ 287 $ 13,992