DERIVATIVE INSTRUMENTS (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) $ / MWh	Dec. 31, 2014 USD ($)	Mar. 31, 2014 USD ($) contracts
Not designated as hedging instrument | Foreign Exchange Forward [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Number of forward currency exchange forward contracts | contracts			2
Notional amount			$ 348,300
Not designated as hedging instrument | Foreign Exchange Forward [Member] | Foreign Currency Gain (Loss) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Income (expense) recognized in earnings on derivatives	$ 0	$ (6,209)
Not designated as hedging instrument | Power Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Liability	37,335	29,368
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	(5,587)	(31,100)
Other Current Liabilities [Member] | Not designated as hedging instrument | Power Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Liability	13,679	8,265
Other Liabilities [Member] | Not designated as hedging instrument | Power Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Liability	$ 23,656	$ 21,103
Minimum [Member] | Power Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Contracted power price notional amount (in USD per megawatts hour) | $ / MWh	41.05
Maximum [Member] | Power Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Contracted power price notional amount (in USD per megawatts hour) | $ / MWh	55.20
Weighted Average [Member] | Power Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Contracted power price notional amount (in USD per megawatts hour) | $ / MWh	43.93