Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Comprehensive Income [Abstract]
Revenue	$ 963,897	$ 948,424	$ 922,003
Operating expenses	825,351	823,496	790,283
Gross margin	138,546	124,928	131,720
Corporate general and administrative expenses	46,066	44,406	39,173
Operating income	92,480	80,522	92,547
Other income (expense):
Interest expense	(4,983)	(5,765)	(6,645)
(Loss) gain on sale of operations, net	(509)	417	1,025
Other income (expense), net	16,500	17,715	(7,087)
Total other income (expense), net	11,008	12,367	(12,707)
Income from continuing operations before income tax expense	103,488	92,889	79,840
Income tax expense	25,141	21,840	18,267
Income from continuing operations	78,347	71,049	61,573
Loss from operations of discontinued operations, net of tax	(48)	(335)	(3)
Net income	$ 78,299	$ 70,714	$ 61,570
Basic:
Continuing operations (in dollars per share)	$ 1.44	$ 1.31	$ 1.13
Discontinued operations (in dollars per share)	0	(0.01)	0
Net income (loss) (in dollars per share)	1.44	1.30	1.13
Diluted:
Continuing operations (in dollars per share)	1.42	1.27	1.09
Discontinued operations (in dollars per share)	(0.01)	(0.01)	0
Net income (loss) (in dollars per share)	$ 1.41	$ 1.26	$ 1.09
Basic weighted average common shares outstanding (in shares)	54,288	54,299	54,561
Diluted weighted average common shares outstanding (in shares)	55,359	55,895	56,487
Comprehensive income:
Net income	$ 78,299	$ 70,714	$ 61,570
Other comprehensive income:
Net unrealized (loss) gain on available-for-sale securities, net of income tax (benefit) expense of $(14), $351 and $(96)	(42)	940	(259)
Net unrealized (loss) on interest rate swaps, net of income tax (benefit) of $(494), $(380) and $(8)	(1,525)	(1,222)	(17)
Foreign currency translation	(22)	(17)	(24)
Total other comprehensive loss	(1,589)	(299)	(300)
Total comprehensive income	$ 76,710	$ 70,415	$ 61,270