Operations by Industry Segment and Geographic Area, External Customers and Long-Lived Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	$ 1,184,739	$ 1,147,431	$ 983,157
Long-lived assets by geographic region	190,396	180,538
Reportable Geographical Components [Member] | United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	932,294	941,273	722,287
Long-lived assets by geographic region	158,683	151,470
Reportable Geographical Components [Member] | Canada [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	65,509	37,539	54,321
Long-lived assets by geographic region	2,893	2,860
Reportable Geographical Components [Member] | Australia and Oceana [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	40,201	29,948	29,995
Reportable Geographical Components [Member] | Africa [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	36,847	31,557	45,671
Reportable Geographical Components [Member] | Other European Countries [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	18,679	19,198	23,867
Reportable Geographical Components [Member] | South America (excluding Brazil) [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	18,562	28,204	32,454
Reportable Geographical Components [Member] | Russia [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	13,609	3,185	8,466
Reportable Geographical Components [Member] | Brazil [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	10,478	4,300	8,376
Long-lived assets by geographic region	11,114	11,288
Reportable Geographical Components [Member] | Other Asian Countries [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	10,286	6,926	9,513
Reportable Geographical Components [Member] | Mexico [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	8,508	13,489	6,990
Reportable Geographical Components [Member] | China [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	6,113	4,595	1,330
Reportable Geographical Components [Member] | Post-Soviet States (excluding Russia) [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	5,951	3,293	8,345
Reportable Geographical Components [Member] | Middle East [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	4,881	3,403	18,995
Reportable Geographical Components [Member] | Japan and Korea [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	4,760	10,825	3,574
Reportable Geographical Components [Member] | West Indies [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	3,421	2,994	1,532
Reportable Geographical Components [Member] | Central America (excluding Mexico) [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	2,929	5,904	4,404
Reportable Geographical Components [Member] | India [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	1,026	318	2,706
Reportable Geographical Components [Member] | Other Foreign Countries [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	685	480	331
Reportable Geographical Components [Member] | Northern Ireland [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets by geographic region	6,342	4,279
Reportable Geographical Components [Member] | South Africa [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets by geographic region	5,684	5,372
Reportable Geographical Components [Member] | Australia [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets by geographic region	4,532	4,234
Reportable Geographical Components [Member] | Germany [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets by geographic region	1,148	1,035
Reportable Geographical Components [Member] | Total Foreign [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	252,445	206,158	$ 260,870
Long-lived assets by geographic region	$ 31,713	$ 29,068