Changes in Accumulated Other Comprehensive Loss by Component - Rollforward (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Changes to accumulated other comprehensive loss by component
Balance at the beginning of the period	$ (6,572)	$ (6,672)
Other comprehensive earning (loss) before reclassifications	711	(189)
Amounts reclassified from accumulated other comprehensive loss	113	155
Balance at the end of the period	(5,748)	(6,706)
Unrealized Net Loss on Interest Rate Swap
Changes to accumulated other comprehensive loss by component
Balance at the beginning of the period	(1,328)	(1,195)
Other comprehensive earning (loss) before reclassifications	711	(189)
Amounts reclassified from accumulated other comprehensive loss	21	60
Balance at the end of the period	(596)	(1,324)
Other Postretirement Benefit Liability
Changes to accumulated other comprehensive loss by component
Balance at the beginning of the period	1,392	937
Amounts reclassified from accumulated other comprehensive loss	(38)	(17)
Balance at the end of the period	1,354	920
Pension Liability
Changes to accumulated other comprehensive loss by component
Balance at the beginning of the period	(6,636)	(6,414)
Amounts reclassified from accumulated other comprehensive loss	130	112
Balance at the end of the period	$ (6,506)	$ (6,302)