Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Statement [Abstract]
Revenues	$ 1,671,389	$ 1,690,176	$ 1,641,959
Operating costs and expenses:
Costs of services provided	1,456,643	1,496,865	1,411,393
Selling, general and administrative expense	166,772	140,344	173,108
Other income (expense):
Investment and other income (loss), net	12,938	(5,427)	9,439
Interest expense	(7,856)	(2,987)	(1,385)
Income before income taxes	53,056	44,553	65,512
Income tax provision	14,670	10,310	16,969
Net income	$ 38,386	$ 34,243	$ 48,543
Per share data:
Basic earnings per common share (in dollars per share)	$ 0.52	$ 0.46	$ 0.65
Diluted earnings per common share (in dollars per share)	$ 0.52	$ 0.46	$ 0.65
Weighted average number of common shares outstanding:
Basic (in shares)	74,288	74,336	74,816
Diluted (in shares)	74,340	74,351	74,962
Comprehensive income:
Net income	$ 38,386	$ 34,243	$ 48,543
Other comprehensive income (loss):
Unrealized gain (loss) on available-for-sale marketable securities, net of taxes	1,633	(7,477)	(1,563)
Total comprehensive income	$ 40,019	$ 26,766	$ 46,980