FAIR VALUE MEASUREMENTS - Summary of Financial Assets at Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale investments included in Reclamation deposits	$ 77,364
Recurring [Member] | Quoted prices in active markets for identical assets - Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale investments included in Restricted investments and bond collateral	35,813	$ 34,057
Available-for-sale investments included in Reclamation deposits	31,545	33,388
Fair value	67,358	67,445
Recurring [Member] | Significant other observable inputs - Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale investments included in Restricted investments and bond collateral	0	0
Available-for-sale investments included in Reclamation deposits	0	0
Fair value	0	0
Fair Value [Member] | Quoted prices in active markets for identical assets - Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale investments included in Restricted investments and bond collateral	35,813
Fair Value [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale investments included in Restricted investments and bond collateral	35,813	34,057
Available-for-sale investments included in Reclamation deposits	31,545	33,388
Fair value	67,358	67,445
Other Current Liabilities and Other Liabilities [Member] | Recurring [Member] | Quoted prices in active markets for identical assets - Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	(646)	(1,981)
Other Current Liabilities and Other Liabilities [Member] | Recurring [Member] | Significant other observable inputs - Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	(37,335)	(29,368)
Other Current Liabilities and Other Liabilities [Member] | Fair Value [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	(37,981)	(31,349)
Power Contract [Member] | Other Current Liabilities and Other Liabilities [Member] | Recurring [Member] | Quoted prices in active markets for identical assets - Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	0	0
Power Contract [Member] | Other Current Liabilities and Other Liabilities [Member] | Recurring [Member] | Significant other observable inputs - Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	(37,335)	(29,368)
Power Contract [Member] | Other Current Liabilities and Other Liabilities [Member] | Fair Value [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	(37,335)	(29,368)
Warrant [Member] | Other Liabilities [Member] | Recurring [Member] | Quoted prices in active markets for identical assets - Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	$ (646)	$ (1,981)
Warrant [Member] | Other Liabilities [Member] | Recurring [Member] | Significant other observable inputs - Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities
Warrant [Member] | Other Liabilities [Member] | Fair Value [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	$ (646)	$ (1,981)