DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES - Fair Value of Each Classification of Derivative Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Derivatives, Fair Value
Fair value	$ 17,699	$ 27,787
Netting Balance Sheet Adjustment	0	0
Total
Derivatives, Fair Value
Fair value	17,699	27,787
Derivatives NOT Designated as Hedging Instruments
Derivatives, Fair Value
Fair value	17,699	(4,207)
Derivatives Designated as Hedging Instruments
Derivatives, Fair Value
Fair value	0	31,994
Total current derivative assets
Derivatives, Fair Value
Fair value	18,220	35,031
Netting Balance Sheet Adjustment	(201)	(72)
Total current derivative assets | Physical fixed price derivative contracts
Derivatives, Fair Value
Fair value	17,934	2,582
Netting Balance Sheet Adjustment	(84)	(63)
Total current derivative assets | Physical index derivative contracts
Derivatives, Fair Value
Fair value	286	455
Netting Balance Sheet Adjustment	(117)	(9)
Total current derivative assets | Interest rate derivatives
Derivatives, Fair Value
Fair value		31,994
Netting Balance Sheet Adjustment		0
Total current derivative assets | Total
Derivatives, Fair Value
Fair value	18,019	34,959
Total current derivative assets | Total | Physical fixed price derivative contracts
Derivatives, Fair Value
Fair value	17,850	2,519
Total current derivative assets | Total | Physical index derivative contracts
Derivatives, Fair Value
Fair value	169	446
Total current derivative assets | Total | Interest rate derivatives
Derivatives, Fair Value
Fair value		31,994
Total current derivative assets | Derivatives NOT Designated as Hedging Instruments
Derivatives, Fair Value
Fair value	18,220	3,037
Total current derivative assets | Derivatives NOT Designated as Hedging Instruments | Physical fixed price derivative contracts
Derivatives, Fair Value
Fair value	17,934	2,582
Total current derivative assets | Derivatives NOT Designated as Hedging Instruments | Physical index derivative contracts
Derivatives, Fair Value
Fair value	286	455
Total current derivative assets | Derivatives NOT Designated as Hedging Instruments | Interest rate derivatives
Derivatives, Fair Value
Fair value		0
Total current derivative assets | Derivatives Designated as Hedging Instruments
Derivatives, Fair Value
Fair value	0	31,994
Total current derivative assets | Derivatives Designated as Hedging Instruments | Physical fixed price derivative contracts
Derivatives, Fair Value
Fair value	0	0
Total current derivative assets | Derivatives Designated as Hedging Instruments | Physical index derivative contracts
Derivatives, Fair Value
Fair value	0	0
Total current derivative assets | Derivatives Designated as Hedging Instruments | Interest rate derivatives
Derivatives, Fair Value
Fair value		31,994
Total non-current derivative assets
Derivatives, Fair Value
Fair value	1,003	0
Netting Balance Sheet Adjustment	0	0
Total non-current derivative assets | Physical fixed price derivative contracts
Derivatives, Fair Value
Fair value	1,003
Netting Balance Sheet Adjustment	0
Total non-current derivative assets | Total
Derivatives, Fair Value
Fair value	1,003	0
Total non-current derivative assets | Total | Physical fixed price derivative contracts
Derivatives, Fair Value
Fair value	1,003
Total non-current derivative assets | Derivatives NOT Designated as Hedging Instruments
Derivatives, Fair Value
Fair value	1,003	0
Total non-current derivative assets | Derivatives NOT Designated as Hedging Instruments | Physical fixed price derivative contracts
Derivatives, Fair Value
Fair value	1,003
Total non-current derivative assets | Derivatives Designated as Hedging Instruments
Derivatives, Fair Value
Fair value	0	0
Total non-current derivative assets | Derivatives Designated as Hedging Instruments | Physical fixed price derivative contracts
Derivatives, Fair Value
Fair value	0
Total current derivative liabilities
Derivatives, Fair Value
Fair value	(1,521)	(7,244)
Netting Balance Sheet Adjustment	201	72
Total current derivative liabilities | Physical fixed price derivative contracts
Derivatives, Fair Value
Fair value	(1,386)	(7,226)
Netting Balance Sheet Adjustment	84	63
Total current derivative liabilities | Physical index derivative contracts
Derivatives, Fair Value
Fair value	(135)	(18)
Netting Balance Sheet Adjustment	117	9
Total current derivative liabilities | Total
Derivatives, Fair Value
Fair value	(1,320)	(7,172)
Total current derivative liabilities | Total | Physical fixed price derivative contracts
Derivatives, Fair Value
Fair value	(1,302)	(7,163)
Total current derivative liabilities | Total | Physical index derivative contracts
Derivatives, Fair Value
Fair value	(18)	(9)
Total current derivative liabilities | Derivatives NOT Designated as Hedging Instruments
Derivatives, Fair Value
Fair value	(1,521)	(7,244)
Total current derivative liabilities | Derivatives NOT Designated as Hedging Instruments | Physical fixed price derivative contracts
Derivatives, Fair Value
Fair value	(1,386)	(7,226)
Total current derivative liabilities | Derivatives NOT Designated as Hedging Instruments | Physical index derivative contracts
Derivatives, Fair Value
Fair value	(135)	(18)
Total current derivative liabilities | Derivatives Designated as Hedging Instruments
Derivatives, Fair Value
Fair value	0	0
Total current derivative liabilities | Derivatives Designated as Hedging Instruments | Physical fixed price derivative contracts
Derivatives, Fair Value
Fair value	0	0
Total current derivative liabilities | Derivatives Designated as Hedging Instruments | Physical index derivative contracts
Derivatives, Fair Value
Fair value	0	0
Total non-current derivative liabilities
Derivatives, Fair Value
Fair value	(3)	0
Netting Balance Sheet Adjustment	0	0
Total non-current derivative liabilities | Physical fixed price derivative contracts
Derivatives, Fair Value
Fair value	(3)
Netting Balance Sheet Adjustment	0
Total non-current derivative liabilities | Total
Derivatives, Fair Value
Fair value	(3)	0
Total non-current derivative liabilities | Total | Physical fixed price derivative contracts
Derivatives, Fair Value
Fair value	(3)
Total non-current derivative liabilities | Derivatives NOT Designated as Hedging Instruments
Derivatives, Fair Value
Fair value	(3)	0
Total non-current derivative liabilities | Derivatives NOT Designated as Hedging Instruments | Physical fixed price derivative contracts
Derivatives, Fair Value
Fair value	(3)
Total non-current derivative liabilities | Derivatives Designated as Hedging Instruments
Derivatives, Fair Value
Fair value	0	$ 0
Total non-current derivative liabilities | Derivatives Designated as Hedging Instruments | Physical fixed price derivative contracts
Derivatives, Fair Value
Fair value	$ 0