Income Taxes - Reconciliation of Income Tax Expense from Statutory Federal Income Tax Rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Difference between total income tax expense and the amount computed by applying the statutory Federal income tax rate to income before taxes:
Income tax at statutory rate	$ 1,171	$ (1,046)	$ 440
State income taxes, net of Federal benefit	317	(185)	66
Oil and mineral depletion	(134)	(180)	(161)
Permanent differences	19	25	82
Excess stock compensation expense	(20)	107	0
U.S. Tax Reform adjustment	0	54	0
Other	(33)	(58)	69
(Benefit) provision for income taxes	$ 1,320	$ (1,283)	$ 496
Effective tax rate	23.80%	41.30%	39.60%