Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Revenues	$ 506,871	$ 491,355	$ 1,512,413	$ 1,366,721
Operating costs and expenses:
Costs of services provided	439,203	426,924	1,346,725	1,179,816
Selling, general and administrative expense	36,713	32,940	104,608	93,141
Other income, net:
Investment and interest, net	2,027	1,439	3,823	4,523
Income before income taxes	32,982	32,930	64,903	98,287
Income tax provision	6,896	9,458	12,931	30,247
Net income	$ 26,086	$ 23,472	$ 51,972	$ 68,040
Per share data:
Basic earnings per common share (in dollars per share)	$ 0.35	$ 0.32	$ 0.70	$ 0.93
Diluted earnings per common share (in dollars per share)	$ 0.35	$ 0.31	$ 0.70	$ 0.92
Weighted average number of common shares outstanding:
Basic (in shares)	74,019	73,461	73,972	73,272
Diluted (in shares)	74,579	74,538	74,598	74,252
Comprehensive income:
Net income	$ 26,086	$ 23,472	$ 51,972	$ 68,040
Other comprehensive income:
Unrealized (loss) gain on available-for-sale marketable securities, net of taxes	(508)	82	(1,549)	1,163
Total comprehensive income	$ 25,578	$ 23,554	$ 50,423	$ 69,203