Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Statement [Abstract]
Revenues	$ 1,641,959	$ 1,760,303	$ 1,840,778
Operating costs and expenses:
Costs of services provided	1,415,082	1,492,317	1,612,877
Selling, general and administrative expense	173,108	150,778	150,022
Other income (expense):
Investment and other income, net	9,439	13,352	10,676
Interest expense	(1,385)	(1,374)	(3,459)
Income before income taxes	61,823	129,186	85,096
Income tax provision	15,960	30,504	20,515
Net income	$ 45,863	$ 98,682	$ 64,581
Per share data:
Basic earnings per common share (in dollars per share)	$ 0.61	$ 1.32	$ 0.87
Diluted earnings per common share (in dollars per share)	$ 0.61	$ 1.32	$ 0.87
Weighted average number of common shares outstanding:
Basic (in shares)	74,816	74,696	74,362
Diluted (in shares)	74,962	74,785	74,590
Comprehensive income:
Net income	$ 45,863	$ 98,682	$ 64,581
Other comprehensive income:
Unrealized (loss) gain on available-for-sale marketable securities, net of taxes	(1,563)	2,644	2,761
Total comprehensive income	$ 44,300	$ 101,326	$ 67,342