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FAIR VALUE MEASUREMENTS Summary of Financial Assets at Fair Value (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]  
Available-for-sale investments included in Reclamation deposits $ 76,715us-gaap_AssetRetirementObligationLegallyRestrictedAssetsFairValue
Fair Value  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]  
Available-for-sale investments included in Reclamation deposits 76,715us-gaap_AssetRetirementObligationLegallyRestrictedAssetsFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
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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 34,591us-gaap_AvailableforsaleSecuritiesRestrictedNoncurrent
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Available-for-sale investments 68,078us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
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 $ 33,487us-gaap_AssetRetirementObligationLegallyRestrictedAssetsFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementBasisAxis
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