Income Taxes - Components of income tax provision (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Income Taxes
Statutory federal income tax rate (as a percent)	34.00%
Income tax at federal rate	$ (1,135)	$ 88
Increase (decrease) resulting from:
State income taxes, net of federal income tax	(123)	12
Expense not deductible for tax		16
Tax credit included in taxable income	93
Other permanent differences	52	4
Capital goods excise tax credit	(273)
Capital goods excise tax debit		(14)
Total income tax provision (credit)	(1,386)	106
Unrecognized tax benefits	0
Interest or penalties related to tax assessments recognized	$ 0	$ 0