DERIVATIVE INSTRUMENTS - FAIR VALUE OF ALL DERIVATIVE INSTRUMENTS (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Fair value Derivative Assets	$ 77,304	$ 99,331
Fair value Derivative Liabilities	89,636	123,348
Commodity Contract
Derivatives, Fair Value [Line Items]
Fair value Derivative Assets	77,304	99,331
Fair value Derivative Liabilities	0	0
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fair value Derivative Assets	77,304	99,331
Fair value Derivative Liabilities	89,636	123,348
Not Designated as Hedging Instrument | Commodity Contract | Derivatives-Energy Related-Current
Derivatives, Fair Value [Line Items]
Fair value Derivative Assets	57,815	83,093
Fair value Derivative Liabilities	66,370	90,708
Not Designated as Hedging Instrument | Commodity Contract | Derivatives-Energy Related-NonCurrent
Derivatives, Fair Value [Line Items]
Fair value Derivative Assets	19,489	16,238
Fair value Derivative Liabilities	10,148	21,697
Not Designated as Hedging Instrument | Interest Rate Contract | Derivatives-Other
Derivatives, Fair Value [Line Items]
Fair value Derivative Assets	0	0
Fair value Derivative Liabilities	$ 13,118	$ 10,943