Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Statement [Abstract]
Revenues	$ 491,355	$ 392,734	$ 1,366,721	$ 1,164,097
Operating costs and expenses:
Costs of services provided	426,924	336,340	1,179,816	998,595
Selling, general and administrative expense	32,940	27,182	93,141	78,192
Other income, net:
Investment and interest	1,439	1,359	4,523	2,548
Income before income taxes	32,930	30,571	98,287	89,858
Income tax provision	9,458	10,860	30,247	32,761
Net income	$ 23,472	$ 19,711	$ 68,040	$ 57,097
Per share data:
Basic earnings per common share (in dollars per share)	$ 0.32	$ 0.27	$ 0.93	$ 0.79
Diluted earnings per common share (in dollars per share)	$ 0.31	$ 0.27	$ 0.92	$ 0.78
Weighted average number of common shares outstanding:
Basic (in shares)	73,461	72,839	73,272	72,718
Diluted (in shares)	74,538	73,592	74,252	73,435
Comprehensive income:
Net income	$ 23,472	$ 19,711	$ 68,040	$ 57,097
Other comprehensive income:
Unrealized gain (loss) on available-for-sale marketable securities, net of taxes	82	(369)	1,163	765
Total comprehensive income	$ 23,554	$ 19,342	$ 69,203	$ 57,862