Income Taxes - Components of income tax provision (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Income Taxes
Statutory federal income tax rate (as a percent)	34.00%	34.00%	34.00%	34.00%
Income tax at federal rate	$ (1,999)	$ 818	$ (3,134)	$ 906
Increase (decrease) resulting from:
State income taxes, net of federal income tax	(233)	134	(356)	146
Expense not deductible for tax		144		160
Tax credit included in taxable income	99		192
Other permanent differences	20	101	72	105
Capital goods excise tax credit	(289)	(237)	(562)	(251)
Total income tax provision (benefit)	(2,402)	960	(3,788)	1,066
Unrecognized tax benefits	0		0
Interest or penalties related to tax assessments recognized	$ 0	$ 0	$ 0	$ 0