Industry Segment and Geographic Areas (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Revenues from External Customers and Long-Lived Assets
Number of segments | segment									1
Revenues	$ 104,413	$ 110,630	$ 113,648	$ 105,041	$ 96,650	$ 100,008	$ 104,176	$ 101,606	$ 433,732	$ 402,440	$ 393,761
Long-lived assets	90,321				93,601				90,321	93,601
Total assets	392,691				391,727				392,691	391,727
United States
Revenues from External Customers and Long-Lived Assets
Revenues									374,650	355,768	347,853
Long-lived assets	54,904				56,980				54,904	56,980
Total assets	293,943				300,688				293,943	300,688
Asia
Revenues from External Customers and Long-Lived Assets
Revenues									9,081	9,133	6,539
Canada
Revenues from External Customers and Long-Lived Assets
Revenues									11,893	10,407	12,587
Europe
Revenues from External Customers and Long-Lived Assets
Revenues									20,147	15,718	15,299
Long-lived assets	15,247				15,806				15,247	15,806
Total assets	74,707				66,862				74,707	66,862
Mexico
Revenues from External Customers and Long-Lived Assets
Revenues									3,603	3,601	3,460
Long-lived assets	20,170				20,815				20,170	20,815
Total assets	$ 24,041				$ 24,177				24,041	24,177
Middle East
Revenues from External Customers and Long-Lived Assets
Revenues									11,318	4,904	5,520
Other
Revenues from External Customers and Long-Lived Assets
Revenues									$ 3,040	$ 2,909	$ 2,503