Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Income Statement [Abstract]
Revenues	$ 407,751	$ 449,150
Operating costs and expenses:
Costs of services provided	336,619	387,156
Selling, general and administrative expense	39,987	30,017
Other income (expense):
Investment and other income, net	2,165	(4,827)
Interest expense	(358)	(368)
Income before income taxes	32,952	26,782
Income tax provision	8,299	6,592
Net income	$ 24,653	$ 20,190
Per share data:
Basic earnings per common share (in dollars per share)	$ 0.33	$ 0.27
Diluted earnings per common share (in dollars per share)	$ 0.33	$ 0.27
Weighted average number of common shares outstanding:
Basic (in shares)	75,003	74,658
Diluted (in shares)	75,224	74,767
Comprehensive income:
Net income	$ 24,653	$ 20,190
Other comprehensive income:
Unrealized loss on available-for-sale marketable securities, net of taxes	(1,271)	(533)
Total comprehensive income	$ 23,382	$ 19,657