Income Taxes - Components of income tax provision (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Income Taxes
Statutory federal income tax rate (as a percent)	21.00%		34.00%
Income tax (benefit) at federal rate	$ (1,523)	$ (1,135)
Increase (decrease) resulting from:
State income taxes, net of federal income tax	(363)	(123)
Tax credit included in taxable income	31	93
Other permanent differences		52
Capital goods excise tax credit	(119)	(273)
Change in valuation allowance	460
Total income tax provision (benefit)	$ (1,514)	(1,386)
Cumulative loss, number of years considered in DTA assessment	3 years
Unrecognized tax benefits	$ 0
Unrecognized tax benefits, income tax penalties and interest expense	$ 0	$ 0