ASSET RETIREMENT OBLIGATIONS, CONTRACTUAL THIRD-PARTY RECLAMATION RECEIVABLES, AND RECLAMATION DEPOSITS - Carrying Value and Estimated Fair Value of Reclamation Deposits (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Reclamation Deposits	$ 72,718,000	$ 71,939,000
Gain on available-for-sale securities held as reclamation deposits	100,000	600,000
Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Reclamation Deposits	72,718,000
Carrying Value [Member] | Cash and cash equivalents [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Reclamation Deposits	37,558,000
Carrying Value [Member] | Held-to-maturity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Reclamation Deposits	19,832,000
Carrying Value [Member] | Time deposits [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Reclamation Deposits	14,289,000
Carrying Value [Member] | Available-For-Sale Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Reclamation Deposits	1,039,000
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Reclamation Deposits	73,899,000
Fair Value [Member] | Cash and cash equivalents [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Reclamation Deposits	37,558,000
Fair Value [Member] | Held-to-maturity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Reclamation Deposits	21,013,000
Fair Value [Member] | Time deposits [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Reclamation Deposits	14,289,000
Fair Value [Member] | Available-For-Sale Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Reclamation Deposits	$ 1,039,000