DERIVATIVE INSTRUMENTS (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Derivative [Line Items]
Gain (loss) on derivative, net	$ 0	$ 2,384
Not Designated as Hedging Instrument | Contract to Purchase Power [Member] | Derivative loss
Derivative [Line Items]
Gain (loss) on derivative, net	0	1,216
Not Designated as Hedging Instrument | Contract to Sell Power | Derivative loss
Derivative [Line Items]
Gain (loss) on derivative, net	0	(3,600)
Not Designated as Hedging Instrument | Power Contract | Derivative loss
Derivative [Line Items]
Gain (loss) on derivative, net	$ 0	$ (2,384)