RESTRICTED INVESTMENTS, RECLAMATION DEPOSITS AND BOND COLLATERAL - Amortized Cost, Gross Unrealized Holding Gains and Losses and Fair Value of Held-to-Maturity Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Restricted Investments and Bond Collateral
Schedule of Available-for-sale Securities [Line Items]
Cost basis	$ 76,281	$ 78,564
Gross unrealized holding gains	465	521
Gross unrealized holding losses	(1,102)	(928)
Fair value	75,644	78,157
Reclamation Deposits
Schedule of Available-for-sale Securities [Line Items]
Cost basis	70,255	70,576
Gross unrealized holding gains	696	617
Gross unrealized holding losses	(940)	(815)
Fair value	70,011	70,378
Restricted Investments
Schedule of Available-for-sale Securities [Line Items]
Cost basis	146,536	149,140
Gross unrealized holding gains	1,161	1,138
Gross unrealized holding losses	(2,042)	(1,743)
Fair value	$ 145,655	$ 148,535