Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
Revenue	$ 224,249	$ 207,723	$ 722,980	$ 660,198
Operating expenses	198,607	184,723	608,459	565,609
Gross margin	25,642	23,000	114,521	94,589
Corporate general and administrative expenses	10,279	7,979	30,300	25,979
Operating income (loss)	15,363	15,021	84,221	68,610
Other (expense) income:
Interest expense	(1,614)	(1,777)	(5,211)	(4,986)
Gain on sale of operations, net			663	45
Other income, net	3,143	2,792	2,544	9,293
Total other income (expense), net	1,529	1,015	(2,004)	4,352
Income from continuing operations before income tax expense	16,892	16,036	82,217	72,962
Income tax expense	3,297	6,172	19,691	26,656
Income from continuing operations	13,595	9,864	62,526	46,306
(Loss) gain from discontinued operations, net of tax	(9)	(206)	17	(776)
Net income	$ 13,586	$ 9,658	$ 62,543	$ 45,530
Basic:
Continuing operations	$ 0.25	$ 0.18	$ 1.15	$ 0.86
Discontinued operations				(0.01)
Net income	0.25	0.18	1.15	0.85
Diluted:
Continuing operations	0.24	0.18	1.11	0.83
Discontinued operations				(0.01)
Net income	$ 0.24	$ 0.18	$ 1.11	$ 0.82
Basic weighted average shares outstanding	54,794	54,142	54,489	53,804
Diluted weighted average shares outstanding	56,740	55,827	56,393	55,641
Comprehensive income:
Net income	$ 13,586	$ 9,658	$ 62,543	$ 45,530
Other comprehensive income, net of tax	55	16	343	233
Comprehensive income	$ 13,641	$ 9,674	$ 62,886	$ 45,763