Changes in Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Changes in Accumulated Other Comprehensive Loss
Balance, beginning of period	$ 394,318	$ 397,841
Balance, end of period	499,194	394,318
Accumulated Other Comprehensive Loss
Changes in Accumulated Other Comprehensive Loss
Balance, beginning of period	(5,468)	(5,441)
Other comprehensive income before reclassifications of gain (loss)	154	5
Amount of gain (loss) reclassified from accumulated other comprehensive income	54	(32)
Total other comprehensive (loss) income	208	(27)
Balance, end of period	(5,260)	(5,468)
Pension Plan
Changes in Accumulated Other Comprehensive Loss
Balance, beginning of period	(5,333)	(4,269)
Other comprehensive income before reclassifications of gain (loss)	21	(1,032)
Amount of gain (loss) reclassified from accumulated other comprehensive income	54	(32)
Total other comprehensive (loss) income	75	(1,064)
Balance, end of period	(5,258)	(5,333)
Derivatives
Changes in Accumulated Other Comprehensive Loss
Balance, beginning of period	(135)	(1,172)
Other comprehensive income before reclassifications of gain (loss)	133	1,037
Total other comprehensive (loss) income	133	1,037
Balance, end of period	$ (2)	$ (135)