COMMITMENTS AND CONTINGENCIES - Lease Obligations and Purchase Commitments (Details) $ in Thousands	Dec. 31, 2016 USD ($)
Lease Obligations
2017	$ 494
2018	309
2019	315
2020	318
2021	257
Thereafter	512
Total	2,205
Operating Leases Future Minimum Payments Due Mining And Surface Rights [Abstract]
2017	2,044
2018	2,337
2019	2,332
2020	2,291
2021	2,261
Thereafter	2,466
Total	13,731
Reclamation Costs
2017	751
2018	415
2019	690
2020	3,540
2021	2,709
Thereafter	2,810
Total	10,915
Total
2017	3,289
More than 5 years	5,788
2018	3,061
2019	3,337
2020	6,149
2021	5,227
Thereafter	5,788
Total	$ 26,851