PROPERTY AND EQUIPMENT (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
PROPERTY AND EQUIPMENT
Total property and equipment	$ 432,144		$ 429,007
Accumulated depreciation and amortization	(199,792)		(194,575)
Property and equipment, net	232,352		234,432
Depreciation and amortization of plant and equipment	6,605	$ 6,633
Cell development costs
PROPERTY AND EQUIPMENT
Total property and equipment	143,749		142,144
Land and improvements
PROPERTY AND EQUIPMENT
Total property and equipment	37,038		36,499
Buildings and improvements
PROPERTY AND EQUIPMENT
Total property and equipment	87,494		87,034
Railcars
PROPERTY AND EQUIPMENT
Total property and equipment	17,299		17,299
Vehicles and other equipment
PROPERTY AND EQUIPMENT
Total property and equipment	125,539		122,697
Construction in progress
PROPERTY AND EQUIPMENT
Total property and equipment	$ 21,025		$ 23,334