ASSET RETIREMENT OBLIGATIONS, CONTRACTUAL THIRD-PARTY RECLAMATION RECEIVABLES, AND RECLAMATION DEPOSITS - Maturities of Held-to-Maturity Securities (Detail) (Reclamation Deposits, USD $) In Thousands, unless otherwise specified	Jun. 30, 2014
Reclamation Deposits
Schedule of Held-to-maturity Securities [Line Items]
Within one year, Amortized cost	$ 1,025
Due in five years or less, Amortized cost	17,341
Due after five years to ten years, Amortized cost	7,289
Due in more than ten years, Amortized cost	6,167
Maturities of held-to-maturity securities, Amortized cost	31,822
Within one year, Fair Value	998
Due in five years or less, Fair Value	17,829
Due after five years to ten years, Fair Value	7,424
Due in more than ten years, Fair Value	6,350
Fair value	$ 32,601