COMMITMENTS AND CONTINGENCIES - Lease Obligations and Purchase Commitments (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Operating lease obligations (office rent)
2019	$ 374
2020	322
2021	263
2022	252
2023	256
Thereafter	143
Total	1,610
Operating lease obligations (mining and surface rights)
2019	2,524
2020	453
2021	460
2022	460
2023	454
Total	4,351
Exploration
2019	15,115
Total	15,115
Construction
2019	6,044
Total	6,044
Reclamation costs
2019	726
2020	1,561
2021	2,890
2022	5,860
2023	141
Thereafter	33,211
Total	44,389
Total
2019	29,658
2020	17,061
2021	45,606
2022	6,572
2023	851
Thereafter	33,354
Total	133,102
Long-term debt
2019	4,875
2020	14,725
2021	41,993
Total	$ 61,593