ASSET RETIREMENT OBLIGATIONS, CONTRACTUAL THIRD-PARTY RECLAMATION RECEIVABLES, AND RECLAMATION DEPOSITS - Carrying Value and Estimated Fair Value of Reclamation Deposits (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015
Asset Retirement Obligation [Line Items]
Reclamation deposits	$ 76,715
Carrying Value
Asset Retirement Obligation [Line Items]
Reclamation deposits	76,715
Carrying Value | Cash and cash equivalents
Asset Retirement Obligation [Line Items]
Reclamation deposits	43,228
Carrying Value | Available-for-sale Securities
Asset Retirement Obligation [Line Items]
Reclamation deposits	33,487
Fair Value
Asset Retirement Obligation [Line Items]
Reclamation deposits	76,715
Fair Value | Cash and cash equivalents | Level 1
Asset Retirement Obligation [Line Items]
Reclamation deposits	43,228
Fair Value | Available-for-sale Securities | Level 1
Asset Retirement Obligation [Line Items]
Reclamation deposits	$ 33,487