Changes in Accumulated Other Comprehensive Loss by Component - Rollforward (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 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	2,058	(656)
Amounts reclassified from accumulated other comprehensive loss	522	450
Balance at the end of the period	(3,992)	(6,878)
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	2,058	(656)
Amounts reclassified from accumulated other comprehensive loss	247	164
Balance at the end of the period	977	(1,687)
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	(115)	(50)
Balance at the end of the period	1,277	887
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	391	336
Balance at the end of the period	$ (6,245)	$ (6,078)