RESTRICTED INVESTMENTS - Amortized Cost, Gross Unrealized Holding Gains and Losses and Fair Value of Held-to-Maturity Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Restricted Investments and Bond Collateral
Schedule of Available-for-sale Securities [Line Items]
Cost basis	$ 55,868	$ 36,715
Gross unrealized holding gains	245	167
Gross unrealized holding losses	(1,171)	(1,069)
Fair value	54,942	35,813
Reclamation Deposits
Schedule of Available-for-sale Securities [Line Items]
Cost basis	48,945	31,977
Gross unrealized holding gains	382	521
Gross unrealized holding losses	(1,074)	(953)
Fair value	48,253	31,545
Restricted Investments
Schedule of Available-for-sale Securities [Line Items]
Cost basis	104,813	68,692
Gross unrealized holding gains	627	688
Gross unrealized holding losses	(2,245)	(2,022)
Fair value	$ 103,195	$ 67,358