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, 2014
Reclamation Deposits [Member]
Schedule of Held-to-maturity Securities [Line Items]
Within one year, Amortized cost	$ 479
Due in five years or less, Amortized cost	17,562
Due after five years to ten years, Amortized cost	4,869
Due in more than ten years, Amortized cost	2,778
Maturities of held-to-maturity securities, Amortized cost	25,688
Within one year, Fair Value	468
Due in five years or less, Fair Value	17,880
Due after five years to ten years, Fair Value	5,023
Due in more than ten years, Fair Value	3,030
Fair value	$ 26,401