DERIVATIVE INSTRUMENTS (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) $ / MWh	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)	Mar. 31, 2014 USD ($) contracts
Derivative [Line Items]
Gain (loss) on derivative, net	$ (5,815)	$ (23,691)	$ (6,717)	$ (29,621)
Not Designated as Hedging Instrument | Foreign Exchange Forward
Derivative [Line Items]
Number of foreign currency exchange forward contracts | contracts						2
Notional amount						$ 348,300
Not Designated as Hedging Instrument | Foreign Exchange Forward | Gain (loss) on foreign exchange
Derivative [Line Items]
Gain (loss) on derivative, net	0	0	0	(6,209)
Not Designated as Hedging Instrument | Power Contract | Derivative loss
Derivative [Line Items]
Gain (loss) on derivative, net	(5,815)	$ (23,691)	(6,717)	$ (29,621)
Not Designated as Hedging Instrument | Power Contract | Other current liabilities
Derivative [Line Items]
Derivative Liability	10,138		10,138		$ 8,265
Not Designated as Hedging Instrument | Power Contract | Other liabilities
Derivative [Line Items]
Derivative Liability	$ 30,175		$ 30,175		$ 21,103
Minimum | Power Contract
Derivative [Line Items]
Contracted power price notional amount | $ / MWh			41.05
Maximum | Power Contract
Derivative [Line Items]
Contracted power price notional amount | $ / MWh			56.33
Weighted Average | Power Contract
Derivative [Line Items]
Contracted power price notional amount | $ / MWh			43.72