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	Mar. 31, 2013
Reclamation Deposits [Member]
Schedule of Held-to-maturity Securities [Line Items]
Within one year, Amortized cost	$ 1,001
Due in five years or less, Amortized cost	6,027
Due after five years to ten years, Amortized cost	4,191
Due in more than ten years, Amortized cost	2,531
Maturities of held-to-maturity securities, Amortized cost	13,750
Within one year, Fair Value	1,005
Due in five years or less, Fair Value	6,477
Due after five years to ten years, Fair Value	4,508
Due in more than ten years, Fair Value	2,840
Maturities of held-to-maturity securities, Fair Value	$ 14,830