DERIVATIVE INSTRUMENTS (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($) $ / MWh	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)
Derivative [Line Items]
Gain (loss) on derivative, net	$ 2,384	$ (2,600)
Contract to Sell Power
Derivative [Line Items]
Optional power delivery, price | $ / MWh	15.26
Not Designated as Hedging Instrument | Contract to Purchase Power [Member] | Derivative loss
Derivative [Line Items]
Gain (loss) on derivative, net	$ 1,216	2,600
Not Designated as Hedging Instrument | Contract to Sell Power | Derivative loss
Derivative [Line Items]
Gain (loss) on derivative, net	(3,600)	0
Not Designated as Hedging Instrument | Contract to Sell Power | Other current assets
Derivative [Line Items]
Derivative asset	9,789		$ 10,240
Not Designated as Hedging Instrument | Contract to Sell Power | Other assets
Derivative [Line Items]
Derivative asset	13,519		9,528
Not Designated as Hedging Instrument | Power Contract | Derivative loss
Derivative [Line Items]
Gain (loss) on derivative, net	(2,384)	$ 2,600
Not Designated as Hedging Instrument | Power Contract | Other current liabilities
Derivative [Line Items]
Derivative liability	13,954		13,382
Not Designated as Hedging Instrument | Power Contract | Other liabilities
Derivative [Line Items]
Derivative liability	$ 19,028		$ 18,384
Minimum | Contract to Sell Power
Derivative [Line Items]
Contracted power price notional amount | $ / MWh	21.33
Minimum | Power Contract
Derivative [Line Items]
Contracted power price notional amount | $ / MWh	41.05
Maximum | Contract to Sell Power
Derivative [Line Items]
Contracted power price notional amount | $ / MWh	24.32
Maximum | Power Contract
Derivative [Line Items]
Contracted power price notional amount | $ / MWh	55.2
Weighted Average | Contract to Sell Power
Derivative [Line Items]
Contracted power price notional amount | $ / MWh	23.87
Weighted Average | Power Contract
Derivative [Line Items]
Contracted power price notional amount | $ / MWh	43.77