Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Components of comprehensive income
Net income	$ 4,231	$ 4,554	$ 6,638	$ 4,262	$ 11,583	$ 5,667	$ 5,278	$ 4,620	$ 19,685	$ 27,148	$ 17,754
Mark to market of foreign exchange hedges									(128)	464	(352)
Deferred income tax (expense) benefit									29	(155)	129
Comprehensive income									$ 19,586	$ 27,457	$ 17,531