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, 2014	Dec. 31, 2013
Asset Retirement Obligation [Line Items]
Reclamation deposits	$ 75,315	$ 74,921
Carrying Value
Asset Retirement Obligation [Line Items]
Reclamation deposits	75,315
Carrying Value | Cash and cash equivalents | Level 1
Asset Retirement Obligation [Line Items]
Reclamation deposits	49,627
Carrying Value | Held-to-maturity securities | Level 2
Asset Retirement Obligation [Line Items]
Reclamation deposits	25,688
Fair Value
Asset Retirement Obligation [Line Items]
Reclamation deposits	76,028
Fair Value | Cash and cash equivalents | Level 1
Asset Retirement Obligation [Line Items]
Reclamation deposits	49,627
Fair Value | Held-to-maturity securities | Level 2
Asset Retirement Obligation [Line Items]
Reclamation deposits	$ 26,401