RESTRICTED INVESTMENTS AND BOND COLLATERAL - Carrying Value and Estimated Fair Value of Restricted Investments and Bond Collateral (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Investment [Line Items]
Restricted investments and bond collateral	$ 144,454	$ 140,807
Restricted Investments and Bond Collateral
Investment [Line Items]
Cash and cash equivalents	64,307	102,539
Time deposits	2,470	2,455
Available-for-sale	77,677	35,813
Restricted investments and bond collateral	144,454	140,807
Reclamation Deposits
Investment [Line Items]
Cash and cash equivalents	2,309	45,819
Time deposits	0	0
Available-for-sale	71,734	31,545
Restricted investments and bond collateral	74,043	77,364
Restricted Investments
Investment [Line Items]
Cash and cash equivalents	66,616	148,358
Time deposits	2,470	2,455
Available-for-sale	149,411	67,358
Restricted investments and bond collateral	$ 218,497	$ 218,171