Segment Information - Schedule of Selected Information for Each Business Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenues from external customers
Revenues from external customers	$ 88,900	$ 79,941	$ 257,103	$ 250,137
Operating income
Operating income	16,482	12,822	47,545	42,646
Interest expense	4,446	4,442	12,794	14,147
Loss on extinguishment of debt	0	0	0	2,161
Other income	(799)	(908)	(2,055)	(2,037)
Income before income taxes	12,835	9,288	36,806	28,375
U.S.
Revenues from external customers
Revenues from external customers	70,255	69,579	215,829	218,706
Operating income
Operating income	12,897	12,243	41,345	40,306
International
Revenues from external customers
Revenues from external customers	18,645	10,362	41,274	31,431
Operating income
Operating income	$ 3,585	$ 579	$ 6,200	$ 2,340