FAIR VALUE MEASUREMENTS Summary of Financial Assets at Fair Value (Details) - Fair Value, Measurements, Recurring $ in Thousands	Mar. 31, 2016 USD ($)
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale investments included in Restricted investments and bond collateral	$ 54,942
Available-for-sale investments included in Reclamation deposits	48,253
Available-for-sale investments	103,195
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale investments included in Restricted investments and bond collateral	0
Available-for-sale investments included in Reclamation deposits	0
Available-for-sale investments	0
Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale investments included in Restricted investments and bond collateral	54,942
Available-for-sale investments included in Reclamation deposits	48,253
Available-for-sale investments	103,195
Other Current Liabilities and Other Liabilities [Member] | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	(557)
Other Current Liabilities and Other Liabilities [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	(39,767)
Other Current Liabilities and Other Liabilities [Member] | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	(40,324)
Other Current Liabilities and Other Liabilities [Member] | Power Contract | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	0
Other Current Liabilities and Other Liabilities [Member] | Power Contract | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	(39,767)
Other Current Liabilities and Other Liabilities [Member] | Power Contract | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	(39,767)
Other liabilities | Warrant | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	(557)
Other liabilities | Warrant | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	0
Other liabilities | Warrant | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	$ (557)