Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Statement [Abstract]
Revenues	$ 435,947	$ 455,606	$ 1,337,126	$ 1,393,818
Operating costs and expenses:
Costs of services provided	365,443	398,404	1,140,116	1,226,154
Selling, general and administrative expense	37,337	33,479	108,819	113,189
Other income (expense):
Investment and other income, net	4,279	733	7,197	7,329
Interest expense	(314)	(740)	(1,062)	(2,579)
Income before income taxes	37,132	23,716	94,326	59,225
Income tax provision	9,488	5,372	23,391	13,539
Net income	$ 27,644	$ 18,344	$ 70,935	$ 45,686
Per share data:
Basic earnings per common share (in dollars per share)	$ 0.37	$ 0.25	$ 0.95	$ 0.61
Diluted earnings per common share (in dollars per share)	$ 0.37	$ 0.25	$ 0.95	$ 0.61
Weighted average number of common shares outstanding:
Basic (in shares)	74,700	74,387	74,684	74,347
Diluted (in shares)	74,777	74,507	74,768	74,615
Comprehensive income:
Net income	$ 27,644	$ 18,344	$ 70,935	$ 45,686
Other comprehensive income:
Unrealized gain on available-for-sale marketable securities, net of taxes	873	457	1,996	2,664
Total comprehensive income	$ 28,517	$ 18,801	$ 72,931	$ 48,350