Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Revenue	$ 241,459	$ 224,238
Operating expenses	192,766	178,117
Gross margin	48,693	46,121
Corporate general and administrative expenses	8,768	10,245
Operating income	39,925	35,876
Other income (expense):
Interest expense	(1,517)	(1,526)
Gain on sale of operations, net	22	101
Other income, net	2,737	2,147
Total other income, net	1,242	722
Income from continuing operations before income tax expense	41,167	36,598
Income tax expense	16,141	14,800
Income from continuing operations	25,026	21,798
Loss from discontinued operations, net of tax	(152)	(30)
Net income	$ 24,874	$ 21,768
Basic:
Continuing operations	$ 0.47	$ 0.42
Net income	0.47	0.42
Diluted:
Continuing operations	0.45	0.41
Net income	$ 0.45	$ 0.41
Basic weighted average shares outstanding	53,293	51,572
Diluted weighted average shares outstanding	55,214	52,745
Comprehensive Income:
Net income	$ 24,874	$ 21,768
Other comprehensive income (loss), net of tax	180	(311)
Comprehensive income	$ 25,054	$ 21,457