DERIVATIVE INSTRUMENTS (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivative [Line Items]
Gain (loss) on derivative, net	$ 0	$ (481)	$ 0	$ 1,904
Not Designated as Hedging Instrument | Contract to Purchase Power [Member] | Derivative loss
Derivative [Line Items]
Gain (loss) on derivative, net	0	2,026	0	3,242
Not Designated as Hedging Instrument | Contract to Sell Power | Derivative loss
Derivative [Line Items]
Gain (loss) on derivative, net	0	(1,545)	0	(5,146)
Not Designated as Hedging Instrument | Power Contract | Derivative loss
Derivative [Line Items]
Gain (loss) on derivative, net	$ 0	$ 481	$ 0	$ (1,904)