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	Sep. 30, 2014	Dec. 31, 2013
Asset Retirement Obligation [Line Items]
Reclamation deposits	$ 77,131	$ 74,921
Carrying Value
Asset Retirement Obligation [Line Items]
Reclamation deposits	77,131
Carrying Value | Cash and cash equivalents
Asset Retirement Obligation [Line Items]
Reclamation deposits	44,248
Carrying Value | Available-for-sale Securities [Member]
Asset Retirement Obligation [Line Items]
Reclamation deposits	32,883
Fair Value
Asset Retirement Obligation [Line Items]
Reclamation deposits	77,131
Fair Value | Cash and cash equivalents | Level 1
Asset Retirement Obligation [Line Items]
Reclamation deposits	44,248
Fair Value | Available-for-sale Securities [Member] | Level 1
Asset Retirement Obligation [Line Items]
Reclamation deposits	$ 32,883