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, 2013	Dec. 31, 2012
Asset Retirement Obligation [Line Items]
Reclamation deposits	$ 73,100	$ 72,718
Carrying Value
Asset Retirement Obligation [Line Items]
Reclamation deposits	73,100
Carrying Value | Cash and cash equivalents | Level 1
Asset Retirement Obligation [Line Items]
Reclamation deposits	44,022
Carrying Value | Held-to-maturity securities | Level 2
Asset Retirement Obligation [Line Items]
Reclamation deposits	13,750
Carrying Value | Time deposits | Level 1
Asset Retirement Obligation [Line Items]
Reclamation deposits	14,289
Carrying Value | Available-for-sale securities | Level 1
Asset Retirement Obligation [Line Items]
Reclamation deposits	1,039
Fair Value
Asset Retirement Obligation [Line Items]
Reclamation deposits	74,180
Fair Value | Cash and cash equivalents | Level 1
Asset Retirement Obligation [Line Items]
Reclamation deposits	44,022
Fair Value | Held-to-maturity securities | Level 2
Asset Retirement Obligation [Line Items]
Reclamation deposits	14,830
Fair Value | Time deposits | Level 1
Asset Retirement Obligation [Line Items]
Reclamation deposits	14,289
Fair Value | Available-for-sale securities | Level 1
Asset Retirement Obligation [Line Items]
Reclamation deposits	$ 1,039