Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Statement [Abstract]
Revenues	$ 455,606	$ 505,500	$ 1,393,818	$ 1,507,649
Operating costs and expenses:
Costs of services provided	398,404	438,262	1,226,154	1,343,545
Selling, general and administrative expense	33,479	36,713	113,189	104,608
Other income (expense):
Investment and other income, net	733	3,239	7,329	7,624
Interest expense	(740)	(782)	(2,579)	(2,217)
Income before income taxes	23,716	32,982	59,225	64,903
Income tax provision	5,372	6,896	13,539	12,931
Net income	$ 18,344	$ 26,086	$ 45,686	$ 51,972
Per share data:
Basic earnings per common share (in dollars per share)	$ 0.25	$ 0.35	$ 0.61	$ 0.70
Diluted earnings per common share (in dollars per share)	$ 0.25	$ 0.35	$ 0.61	$ 0.70
Weighted average number of common shares outstanding:
Basic (in shares)	74,387	74,019	74,347	73,972
Diluted (in shares)	74,507	74,579	74,615	74,598
Comprehensive income:
Net income	$ 18,344	$ 26,086	$ 45,686	$ 51,972
Other comprehensive income:
Unrealized gain (loss) on available-for-sale marketable securities, net of taxes	457	(508)	2,664	(1,549)
Total comprehensive income	$ 18,801	$ 25,578	$ 48,350	$ 50,423