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	Jun. 30, 2014	Dec. 31, 2013
Asset Retirement Obligation [Line Items]
Reclamation deposits	$ 75,911	$ 74,921
Carrying Value
Asset Retirement Obligation [Line Items]
Reclamation deposits	75,911
Carrying Value | Cash and cash equivalents | Level 1
Asset Retirement Obligation [Line Items]
Reclamation deposits	44,089
Carrying Value | Held-to-maturity securities | Level 2
Asset Retirement Obligation [Line Items]
Reclamation deposits	31,822
Fair Value
Asset Retirement Obligation [Line Items]
Reclamation deposits	76,690
Fair Value | Cash and cash equivalents | Level 1
Asset Retirement Obligation [Line Items]
Reclamation deposits	44,089
Fair Value | Held-to-maturity securities | Level 2
Asset Retirement Obligation [Line Items]
Reclamation deposits	$ 32,601