Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Income Statement [Abstract]
Revenues	$ 414,488	$ 415,590	$ 1,266,156	$ 1,221,512
Operating costs and expenses:
Costs of services provided	376,894	364,822	1,129,526	1,037,852
Selling, general and administrative expense	35,803	38,780	100,820	128,818
Other (expense) income:
Investment and other income (loss), net	1	462	(9,034)	6,311
Interest expense	(799)	(329)	(1,752)	(1,017)
Income before income taxes	993	12,121	25,024	60,136
Income tax provision	671	2,581	6,553	16,378
Net income	$ 322	$ 9,540	$ 18,471	$ 43,758
Per share data:
Basic earnings per share (in dollars per share)	$ 0.00	$ 0.13	$ 0.25	$ 0.58
Diluted earnings per share (in dollars per share)	$ 0.00	$ 0.13	$ 0.25	$ 0.58
Weighted average number of common shares outstanding:
Basic (in shares)	74,340	74,943	74,334	74,983
Diluted (in shares)	74,348	75,076	74,346	75,170
Comprehensive income:
Net income	$ 322	$ 9,540	$ 18,471	$ 43,758
Other comprehensive loss:
Unrealized loss on available-for-sale marketable securities, net of taxes	(2,293)	(1,014)	(9,507)	(1,582)
Total comprehensive (loss) income	$ (1,971)	$ 8,526	$ 8,964	$ 42,176