FAIR VALUE MEASUREMENTS Summary of Financial Assets at Fair Value (Details) - Fair Value, Measurements, Recurring $ in Thousands	Mar. 31, 2017 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	$ 79,180
Available-for-sale investments included in Reclamation deposits	72,638
Available-for-sale investments	151,818
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	23,308
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	79,180
Available-for-sale investments included in Reclamation deposits	72,638
Available-for-sale investments	175,126
Other Current Assets and Other Assets [Member] | Power Contract | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	0
Other Current Assets and Other Assets [Member] | Power Contract | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	23,308
Other Current Assets and Other Assets [Member] | Power Contract | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	23,308
Other current liabilities and other liabilities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability	520
Other current liabilities and other liabilities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability	32,982
Other current liabilities and other liabilities | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability	33,502
Other current liabilities and other liabilities | 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 | Power Contract | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability	32,982
Other current liabilities and other liabilities | Power Contract | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability	32,982
Other liabilities | Warrant | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability	520
Other liabilities | Warrant | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability
Other liabilities | Warrant | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability	$ 520