Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement Of Income And Comprehensive Income [Abstract]
Revenue	$ 207,723	$ 199,794	$ 660,198	$ 621,047
Operating expenses	184,723	174,069	565,609	526,182
Gross margin	23,000	25,725	94,589	94,865
Corporate general and administrative expenses	7,979	8,679	25,979	27,270
Operating income	15,021	17,046	68,610	67,595
Other (expense) income:
Interest expense	(1,777)	(1,760)	(4,986)	(5,019)
Gain on sale of operations, net		329	45	480
Other income, net	2,792	2,632	9,293	5,482
Total other income, net	1,015	1,201	4,352	943
Income from continuing operations before income tax expense	16,036	18,247	72,962	68,538
Income tax expense	6,172	7,260	26,656	27,366
Income from continuing operations	9,864	10,987	46,306	41,172
Loss from discontinued operations, net of tax	(206)	(133)	(776)	(421)
Net income	$ 9,658	$ 10,854	$ 45,530	$ 40,751
Basic:
Continuing operations	$ 0.18	$ 0.21	$ 0.86	$ 0.79
Discontinued operations			(0.01)	(0.01)
Net income	0.18	0.21	0.85	0.78
Diluted:
Continuing operations	0.18	0.20	0.83	0.77
Discontinued operations			(0.01)	(0.01)
Net income	$ 0.18	$ 0.20	$ 0.82	$ 0.76
Basic weighted average shares outstanding	54,142	52,648	53,804	52,086
Diluted weighted average shares outstanding	55,827	53,846	55,641	53,320
Comprehensive Income:
Net income	$ 9,658	$ 10,854	$ 45,530	$ 40,751
Other comprehensive income (loss), net of tax	16	150	233	(291)
Comprehensive income	$ 9,674	$ 11,004	$ 45,763	$ 40,460