Income Taxes - Components of income tax provision (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Current
Federal
State and local	(741)	(601)	(941)
Current	(741)	(601)	(941)
Deferred
Federal	1,160	1,726	6,081
State and local	172	232	770
Deferred	$ 1,332	1,958	6,851
Statutory federal income tax rate (as a percent)	34.00%
Income tax at federal rate	$ 577	835	4,763
Increase (decrease) resulting from:
State income taxes, net of federal income tax	101	104	592
Permanent difference for compensation limitation		637	595
Expense not deductible for tax	292	98	415
Tax credit included in taxable income	283	252	327
Other permanent differences	20	110	47
Capital goods excise tax credit	(741)	(601)	(942)
Other, net	74	54	51
Change in valuation allowance	(15)	(132)	62
Total income tax provision	591	1,357	5,910
Unrecognized tax benefits	0	0	0
Interest or penalties related to tax assessments recognized	$ 0	$ 0	$ 0