Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Statement [Abstract]
Revenues	$ 415,590	$ 435,947	$ 1,221,512	$ 1,337,126
Operating costs and expenses:
Costs of services provided	364,822	365,443	1,037,852	1,140,116
Selling, general and administrative expense	38,780	37,337	128,818	108,819
Other income (expense):
Investment and other income, net	462	4,279	6,311	7,197
Interest expense	(329)	(314)	(1,017)	(1,062)
Income before income taxes	12,121	37,132	60,136	94,326
Income tax provision	2,581	9,488	16,378	23,391
Net income	$ 9,540	$ 27,644	$ 43,758	$ 70,935
Per share data:
Basic earnings per common share (in dollars per share)	$ 0.13	$ 0.37	$ 0.58	$ 0.95
Diluted earnings per common share (in dollars per share)	$ 0.13	$ 0.37	$ 0.58	$ 0.95
Weighted average number of common shares outstanding:
Basic (in shares)	74,943	74,700	74,983	74,684
Diluted (in shares)	75,076	74,777	75,170	74,768
Comprehensive income:
Net income	$ 9,540	$ 27,644	$ 43,758	$ 70,935
Other comprehensive income:
Unrealized (loss) gain on available-for-sale marketable securities, net of taxes	(1,014)	873	(1,582)	1,996
Total comprehensive income	$ 8,526	$ 28,517	$ 42,176	$ 72,931