Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Income Statement [Abstract]
Revenues	$ 386,556	$ 355,356	$ 771,363	$ 710,602
Operating costs and expenses:
Costs of services provided	332,211	304,217	662,255	608,153
Selling, general and administrative expense	25,664	25,124	51,010	51,887
Other income, net:
Investment and interest	1,002	242	1,189	749
Income before income taxes	29,683	26,257	59,287	51,311
Income tax provision	10,923	9,969	21,901	19,507
Net income	$ 18,760	$ 16,288	$ 37,386	$ 31,804
Per share data:
Basic earnings per common share (in dollars per share)	$ 0.26	$ 0.23	$ 0.52	$ 0.44
Diluted earnings per common share (in dollars per share)	$ 0.26	$ 0.23	$ 0.51	$ 0.44
Weighted average number of common shares outstanding:
Basic (in shares)	72,568	71,657	72,466	71,563
Diluted (in shares)	73,316	72,286	73,165	72,223
Comprehensive income:
Net income	$ 18,760	$ 16,288	$ 37,386	$ 31,804
Other comprehensive income:
Unrealized gain (loss) on available for sale marketable securities, net of taxes	578	(20)	1,133	(17)
Total comprehensive income	$ 19,338	$ 16,268	$ 38,519	$ 31,787