INCOME TAXES (Deferred Tax Assets Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets (liabilities):
Property, plant, equipment and mineral properties, net	$ 133,586	$ 138,855
Federal and state net operating loss carryforwards	63,194	65,779
Other	2,839	2,888
Asset retirement obligation	5,763	5,950
Deferred revenue	4,371	3,035
R&D Credits	1,870	1,870
Total Deferred Tax Assets	211,623	218,377
Valuation allowance	(211,623)	(218,377)
Deferred tax asset, net	$ 0	$ 0