Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Income Statement [Abstract]
Revenues	$ 404,490	$ 384,807
Operating costs and expenses:
Costs of services provided	345,570	330,044
Selling, general and administrative expense	28,210	25,346
Other income, net:
Investment and interest	1,569	187
Income before income taxes	32,279	29,604
Income tax provision	10,262	10,978
Net income	$ 22,017	$ 18,626
Per share data:
Basic earnings per common share (in dollars per share)	$ 0.30	$ 0.26
Diluted earnings per common share (in dollars per share)	$ 0.30	$ 0.26
Weighted average number of common shares outstanding:
Basic (in shares)	73,074	72,364
Diluted (in shares)	73,946	73,014
Comprehensive income:
Net income	$ 22,017	$ 18,626
Other comprehensive income:
Unrealized gain on available for sale marketable securities, net of taxes	479	555
Total comprehensive income	$ 22,496	$ 19,181