Industry Segments and Geographic Information Industry Segments and Geographic Information 2 (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
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	$ 282,471	$ 256,851	$ 260,072	$ 199,381	$ 200,081	$ 183,677	$ 179,931	$ 159,409	$ 998,775	$ 723,098	$ 617,391
Transfers between geographic areas									(3,157)	(2,025)	(93)
Identifiable assets	1,385,527				1,308,614				1,385,527	1,308,614	1,315,540
UNITED STATES
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from Contract with Customer, Including Assessed Tax									791,389	545,964	458,227
Transfers between geographic areas									0	0	0
Identifiable assets	1,211,759				1,131,650				1,211,759	1,131,650	1,132,986
Canada and Mexico [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from Contract with Customer, Including Assessed Tax									41,524	36,074	34,594
Transfers between geographic areas									0	0	0
Identifiable assets	59,355				62,537				59,355	62,537	64,163
South America [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from Contract with Customer, Including Assessed Tax									25,781	28,040	20,480
Transfers between geographic areas									0	0	0
Identifiable assets	25,122				23,352				25,122	23,352	21,354
Europe [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from Contract with Customer, Including Assessed Tax									93,262	80,721	71,882
Transfers between geographic areas									3,157	2,025	93
Identifiable assets	57,807				61,000				57,807	61,000	53,713
Africa [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from Contract with Customer, Including Assessed Tax									12,367	700	10,345
Transfers between geographic areas									0	0	0
Identifiable assets	14,772				3,696				14,772	3,696	5,711
Asia and other [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from Contract with Customer, Including Assessed Tax									34,452	31,599	21,863
Transfers between geographic areas									0	0	0
Identifiable assets	$ 16,712				$ 26,379				$ 16,712	$ 26,379	$ 37,613