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	Mar. 31, 2017	Dec. 31, 2016
Restricted Investments and Bond Collateral
Schedule of Available-for-sale Securities [Line Items]
Cost basis	$ 79,767	$ 76,558
Gross unrealized holding gains	394	251
Gross unrealized holding losses	(981)	(1,229)
Fair value	79,180	75,580
Reclamation Deposits
Schedule of Available-for-sale Securities [Line Items]
Cost basis	72,915	72,381
Gross unrealized holding gains	622	453
Gross unrealized holding losses	(899)	(1,145)
Fair value	72,638	71,689
Restricted Investments
Schedule of Available-for-sale Securities [Line Items]
Cost basis	152,682	148,939
Gross unrealized holding gains	1,016	704
Gross unrealized holding losses	(1,880)	(2,374)
Fair value	$ 151,818	$ 147,269