Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Statement [Abstract]
Revenues	$ 462,101	$ 501,587	$ 938,212	$ 1,002,149
Operating costs and expenses:
Costs of services provided	400,485	436,287	827,750	905,283
Selling, general and administrative expense	38,609	34,118	79,710	67,895
Other income (expense):
Investment and other income, net	1,393	2,845	6,596	4,385
Interest expense	(783)	(711)	(1,839)	(1,435)
Income before income taxes	23,617	33,316	35,509	31,921
Income tax provision	5,431	7,502	8,167	6,035
Net income	$ 18,186	$ 25,814	$ 27,342	$ 25,886
Per share data:
Basic earnings per common share (in dollars per share)	$ 0.24	$ 0.35	$ 0.37	$ 0.35
Diluted earnings per common share (in dollars per share)	$ 0.24	$ 0.35	$ 0.37	$ 0.35
Weighted average number of common shares outstanding:
Basic (in shares)	74,352	73,982	74,327	73,947
Diluted (in shares)	74,619	74,487	74,669	74,606
Comprehensive income:
Net income	$ 18,186	$ 25,814	$ 27,342	$ 25,886
Other comprehensive income:
Unrealized gain (loss) on available-for-sale marketable securities, net of taxes	791	97	2,207	(1,041)
Total comprehensive income	$ 18,977	$ 25,911	$ 29,549	$ 24,845