ASSET RETIREMENT OBLIGATIONS, CONTRACTUAL THIRD-PARTY RECLAMATION RECEIVABLES, AND RECLAMATION DEPOSITS - Maturities of Available-for-Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Within one year, Amortized cost	$ 452
Due in five years or less, Amortized cost	17,142
Due after five years to ten years, Amortized cost	7,142
Due in more than ten years, Amortized cost	8,194
Reclamation Deposits [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, amortized cost	32,930	0
Within one year, Fair Value	475
Due in five years or less, Fair Value	17,282
Due after five years to ten years, Fair Value	7,183
Due in more than ten years, Fair Value	8,448
Fair value	$ 33,388