FAIR VALUE MEASUREMENTS Summary of Financial Assets at Fair Value (Details) - Fair Value, Measurements, Recurring $ in Thousands	Sep. 30, 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	$ 77,677
Available-for-sale investments included in Reclamation deposits	71,734
Available-for-sale investments	149,411
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	77,677
Available-for-sale investments included in Reclamation deposits	71,734
Available-for-sale investments	149,411
Other Current Liabilities and Other Liabilities [Member] | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	610
Other Current Liabilities and Other Liabilities [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	39,164
Other Current Liabilities and Other Liabilities [Member] | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	39,774
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	39,164
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	39,164
Other liabilities | Warrant | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	610
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	$ 610