RESTRICTED INVESTMENTS AND BOND COLLATERAL - Carrying Value and Estimated Fair Value of Restricted Investments and Bond Collateral (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Investment [Line Items]
Restricted investments and bond collateral	$ 145,642	$ 144,913
Restricted Investments and Bond Collateral
Investment [Line Items]
Cash and cash equivalents	63,989	66,860
Time deposits	2,473	2,473
Available-for-sale	79,180	75,580
Restricted investments and bond collateral	145,642	144,913
Reclamation Deposits
Investment [Line Items]
Cash and cash equivalents	2,873	2,673
Time deposits	0	0
Available-for-sale	72,638	71,689
Restricted investments and bond collateral	75,511	74,362
Restricted Investments
Investment [Line Items]
Cash and cash equivalents	66,862	69,533
Time deposits	2,473	2,473
Available-for-sale	151,818	147,269
Restricted investments and bond collateral	$ 221,153	$ 219,275