DERIVATIVE INSTRUMENTS (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) $ / MWh	Dec. 31, 2015 USD ($)
Contract to Sell Power [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Optional Power Delivery, Price | $ / MWh	15.26
Not designated as hedging instrument | Contract to Sell Power [Member] | Gain (Loss) on Derivative Instruments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Income (expense) recognized in earnings on derivatives | $	$ (19,768)	$ 0
Not designated as hedging instrument | Contract to Purchase Power [Member] | Gain (Loss) on Derivative Instruments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Income (expense) recognized in earnings on derivatives | $	(4,287)	5,587
Not designated as hedging instrument | Power Contract [Member] | Gain (Loss) on Derivative Instruments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Income (expense) recognized in earnings on derivatives | $	(24,055)	5,587
Other Current Liabilities [Member] | Not designated as hedging instrument | Contract to Purchase Power [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Liability | $	13,382	13,679
Other Liabilities [Member] | Not designated as hedging instrument | Contract to Purchase Power [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Liability | $	18,384	23,656
Other Current Assets [Member] | Not designated as hedging instrument | Contract to Sell Power [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Asset | $	10,240	0
Other Assets [Member] | Not designated as hedging instrument | Contract to Sell Power [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Asset | $	$ 9,528	$ 0
Minimum [Member] | Contract to Sell Power [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Contracted power price notional amount (in USD per megawatts hour) | $ / MWh	21.33
Minimum [Member] | Contract to Purchase Power [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Contracted power price notional amount (in USD per megawatts hour) | $ / MWh	41.05
Maximum [Member] | Contract to Sell Power [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Contracted power price notional amount (in USD per megawatts hour) | $ / MWh	24.32
Maximum [Member] | Contract to Purchase Power [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Contracted power price notional amount (in USD per megawatts hour) | $ / MWh	55.20
Weighted Average [Member] | Contract to Sell Power [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Contracted power price notional amount (in USD per megawatts hour) | $ / MWh	23.88
Weighted Average [Member] | Contract to Purchase Power [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Contracted power price notional amount (in USD per megawatts hour) | $ / MWh	44.33