RESTRICTED INVESTMENTS AND BOND COLLATERAL - Amortized Cost, Gross Unrealized Holding Gains and Losses and Fair Value of Held-to-Maturity Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Restricted Investments and Bond Collateral
Schedule of Available-for-sale Securities [Line Items]
Cost basis	$ 78,410	$ 36,715
Gross unrealized holding gains	273	167
Gross unrealized holding losses	(1,006)	(1,069)
Fair value	77,677	35,813
Reclamation Deposits
Schedule of Available-for-sale Securities [Line Items]
Cost basis	72,068	31,977
Gross unrealized holding gains	551	521
Gross unrealized holding losses	(885)	(953)
Fair value	71,734	31,545
Restricted Investments
Schedule of Available-for-sale Securities [Line Items]
Cost basis	150,478	68,692
Gross unrealized holding gains	824	688
Gross unrealized holding losses	(1,891)	(2,022)
Fair value	$ 149,411	$ 67,358