Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Revenues	$ 25,754	$ 21,285	$ 51,381	$ 41,521
Costs and expenses
Operating costs	17,966	15,808	36,892	30,706
Selling, general and administrative	3,938	3,435	7,417	7,153
Depreciation and amortization	6,579	6,335	13,170	12,591
Asset impairment, net	0	546	(35)	675
(Gain) loss on disposition of assets, net	(333)	745	(415)	1,573
Total costs and expenses	28,150	26,869	57,029	52,698
Operating loss	(2,396)	(5,584)	(5,648)	(11,177)
Interest expense	(938)	(686)	(1,881)	(1,316)
Loss before income taxes	(3,334)	(6,270)	(7,529)	(12,493)
Income tax (benefit) expense	(21)	34	(70)	80
Net loss	$ (3,313)	$ (6,304)	$ (7,459)	$ (12,573)
Loss per share:
Basic and diluted (usd per share)	$ (0.09)	$ (0.17)	$ (0.20)	$ (0.33)
Weighted average number of common shares outstanding:
Basic and diluted (in shares)	38,253	37,679	38,188	37,613