Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Income Statement [Abstract]
Revenues	$ 398,171	$ 452,029	$ 805,922	$ 901,179
Operating costs and expenses:
Costs of services provided	336,411	387,517	673,030	774,673
Selling, general and administrative expense	50,051	41,465	90,038	71,482
Other income (expense):
Investment and other income, net	3,684	7,745	5,849	2,918
Interest expense	(330)	(380)	(688)	(748)
Income before income taxes	15,063	30,412	48,015	57,194
Income tax provision	5,498	7,311	13,797	13,903
Net income	$ 9,565	$ 23,101	$ 34,218	$ 43,291
Per share data:
Basic earnings per common share (in dollars per share)	$ 0.13	$ 0.31	$ 0.46	$ 0.58
Diluted earnings per common share (in dollars per share)	$ 0.13	$ 0.31	$ 0.45	$ 0.58
Weighted average number of common shares outstanding:
Basic (in shares)	75,005	74,695	75,004	74,676
Diluted (in shares)	75,212	74,761	75,218	74,764
Comprehensive income:
Net income	$ 9,565	$ 23,101	$ 34,218	$ 43,291
Other comprehensive income:
Unrealized gain (loss) on available-for-sale marketable securities, net of taxes	703	1,656	(568)	1,123
Total comprehensive income	$ 10,268	$ 24,757	$ 33,650	$ 44,414