Unaudited Condensed Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Statement [Abstract]
Revenues	$ 65,761	$ 65,278	$ 123,221	$ 121,221
Cost and expenses
Cost of sales (exclusive of items shown separately below)	43,219	47,860	84,225	92,191
Asset retirement obligation accretion	128	124	256	247
Depreciation and amortization	4,822	2,955	8,938	5,393
Selling, general and administrative	4,703	3,692	8,664	7,123
Total cost and expenses	52,872	54,631	102,083	104,954
Operating income	12,889	10,647	21,138	16,267
Other income	194	513	492	1,002
Interest expense, net	(302)	(314)	(609)	(414)
Income before taxes	12,781	10,846	21,021	16,855
Income tax expense	2,168	642	3,525	1,385
Net income	$ 10,613	$ 10,204	$ 17,496	$ 15,470
Earnings Per Share
Basic earnings (in dollars per share)	$ 0.26	$ 0.25	$ 0.43	$ 0.39
Diluted earnings (in dollars per share)	$ 0.26	$ 0.25	$ 0.43	$ 0.38
Weighted average common shares outstanding
Basic weighted average shares outstanding (in shares)	40,869	40,082	40,737	39,994
Diluted weighted average shares outstanding (in shares)	40,965	40,340	40,810	40,242