Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components of comprehensive income
Net income	$ 11,583	$ 5,667	$ 5,278	$ 4,620	$ 3,920	$ 6,081	$ 3,687	$ 4,066	$ 27,148	$ 17,754	$ 12,886
Minimum pension liability adjustments											946
Mark to market of foreign exchange hedges									464	(352)
Reclassification to interest expense											678
Mark to market of interest rate hedges											(15)
Deferred income tax (expense) benefit									(155)	129	(585)
Comprehensive (loss) income									$ 27,457	$ 17,531	$ 13,910