ASSET RETIREMENT OBLIGATIONS - Estimated Payments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
ASSET RETIREMENT OBLIGATIONS
Surety bonds outstanding to performance of reclamation obligations	$ 153,500	$ 142,300
Estimated payments of asset retirement obligations:
2016	1,251
2017	1,073
2018	4,582
2019	4,832
2020	5,093
Thereafter	211,654
Aggregate undiscounted asset retirement obligations	228,485
Effect of discounting	(104,800)	(88,800)
Total asset retirement obligations	123,685	93,140	$ 93,140	$ 82,898
Less: current portion	(1,251)
Asset retirement obligations	$ 122,434	$ 91,085