ASSET RETIREMENT OBLIGATIONS - Estimated Payments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Estimated payments of asset retirement obligations:
2017	$ 435
2018	385
2019	158
2020	147
2021	80
Thereafter	235,237
Aggregate undiscounted asset retirement obligations	236,442
Effect of discounting	(110,741)	$ (104,800)
Total asset retirement obligations	125,701	123,685	$ 93,140
Less: current portion	(435)
Asset retirement obligations	125,266	122,434
Surety bonds outstanding to performance of reclamation obligations	$ 171,800	$ 153,500