Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Income Statement [Abstract]
Revenues	$ 470,876	$ 386,556	$ 875,366	$ 771,363
Operating costs and expenses:
Costs of services provided	407,322	332,211	752,892	662,255
Selling, general and administrative expense	31,991	25,664	60,201	51,010
Other income, net:
Investment and interest	1,515	1,002	3,084	1,189
Income before income taxes	33,078	29,683	65,357	59,287
Income tax provision	10,527	10,923	20,789	21,901
Net income	$ 22,551	$ 18,760	$ 44,568	$ 37,386
Per share data:
Basic earnings per common share (in dollars per share)	$ 0.31	$ 0.26	$ 0.61	$ 0.52
Diluted earnings per common share (in dollars per share)	$ 0.30	$ 0.26	$ 0.60	$ 0.51
Weighted average number of common shares outstanding:
Basic (in shares)	73,276	72,568	73,176	72,466
Diluted (in shares)	74,269	73,316	74,108	73,165
Comprehensive income:
Net income	$ 22,551	$ 18,760	$ 44,568	$ 37,386
Other comprehensive income:
Unrealized gain on available-for-sale marketable securities, net of taxes	602	578	1,081	1,133
Total comprehensive income	$ 23,153	$ 19,338	$ 45,649	$ 38,519