Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Revenue	$ 266,090	$ 241,459
Operating expenses	204,750	192,766
Gross margin	61,340	48,693
Corporate general and administrative expenses	10,028	8,768
Operating income	51,312	39,925
Other (expense) income:
Interest expense	(1,780)	(1,517)
Gain on sale of operations, net	663	22
Other (expense) income, net	(1,229)	2,737
Total other (expense) income, net	(2,346)	1,242
Income from continuing operations before income tax expense	48,966	41,167
Income tax expense	13,156	16,141
Income from continuing operations	35,810	25,026
Gain (loss) from discontinued operations, net of tax	41	(152)
Net income	$ 35,851	$ 24,874
Basic:
Continuing operations	$ 0.66	$ 0.47
Net income	0.66	0.47
Diluted:
Continuing operations	0.64	0.45
Net income	$ 0.64	$ 0.45
Basic weighted average shares outstanding	54,071	53,293
Diluted weighted average shares outstanding	55,924	55,214
Comprehensive income:
Net income	$ 35,851	$ 24,874
Other comprehensive income, net of tax	114	180
Comprehensive income	$ 35,965	$ 25,054