ASSET RETIREMENT OBLIGATIONS, CONTRACTUAL THIRD-PARTY RECLAMATION RECEIVABLES, AND RECLAMATION DEPOSITS - Carrying Value and Estimated Fair Value of Reclamation Deposits (Detail) $ in Thousands	Sep. 30, 2015 USD ($)
Asset Retirement Obligation [Line Items]
Reclamation deposits	$ 77,425
Carrying Value
Asset Retirement Obligation [Line Items]
Reclamation deposits	77,425
Carrying Value | Cash and cash equivalents
Asset Retirement Obligation [Line Items]
Reclamation deposits	46,544
Carrying Value | Available-for-sale Securities
Asset Retirement Obligation [Line Items]
Reclamation deposits	30,881
Fair Value
Asset Retirement Obligation [Line Items]
Reclamation deposits	77,425
Fair Value | Cash and cash equivalents | Level 1
Asset Retirement Obligation [Line Items]
Reclamation deposits	46,544
Fair Value | Available-for-sale Securities | Level 1
Asset Retirement Obligation [Line Items]
Reclamation deposits	$ 30,881