FAIR VALUE MEASUREMENTS Summary of Financial Assets at Fair Value (Details) $ in Thousands	Sep. 30, 2015 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale investments included in Reclamation deposits	$ 77,425
Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale investments included in Reclamation deposits	77,425
Fair Value \| 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	34,854
Available-for-sale investments	65,735
Available-for-sale Securities \| Fair Value \| Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale investments included in Reclamation deposits	$ 30,881

Carrying amount of an asset that is legally restricted for purposes of settling an asset retirement obligation.

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-Name Accounting Standards Codification

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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