ASSET RETIREMENT OBLIGATIONS, CONTRACTUAL THIRD-PARTY RECLAMATION RECEIVABLES, AND RECLAMATION DEPOSITS - Maturities of Held-to-Maturity Securities (Detail) (Reclamation Deposits [Member], USD $) In Thousands, unless otherwise specified	Sep. 30, 2013
Reclamation Deposits [Member]
Schedule of Held-to-maturity Securities [Line Items]
Within one year, Amortized cost	$ 26
Due in five years or less, Amortized cost	6,664
Due after five years to ten years, Amortized cost	3,118
Due in more than ten years, Amortized cost	1,911
Maturities of held-to-maturity securities, Amortized cost	11,719
Within one year, Fair Value	28
Due in five years or less, Fair Value	7,011
Due after five years to ten years, Fair Value	3,236
Due in more than ten years, Fair Value	2,149
Maturities of held-to-maturity securities, Fair Value	$ 12,424