ASSET RETIREMENT OBLIGATIONS - Estimated Payments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Estimated payments of asset retirement obligations:
2018	$ 3,850
2019	454
2020	433
2022	1,256
Thereafter	238,604
Aggregate undiscounted asset retirement obligations	244,597
Effect of discounting	(113,997)	$ (110,700)
Total asset retirement obligations	130,600	125,701	$ 123,685
Less: current portion	(3,850)
Asset retirement obligations	126,750	125,266
Surety bonds outstanding to performance of reclamation obligations	$ 172,900	$ 171,800