Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Income Statement [Abstract]
Revenues	$ 449,150	$ 476,111
Operating costs and expenses:
Costs of services provided	387,156	427,265
Selling, general and administrative expense	30,017	41,101
Other (expense) income:
Investment and other income, net	(4,827)	5,203
Interest expense	(368)	(1,056)
Income before income taxes	26,782	11,892
Income tax provision	6,592	2,736
Net income	$ 20,190	$ 9,156
Per share data:
Basic earnings per common share (in dollars per share)	$ 0.27	$ 0.12
Diluted earnings per common share (in dollars per share)	$ 0.27	$ 0.12
Weighted average number of common shares outstanding:
Basic (in shares)	74,658	74,301
Diluted (in shares)	74,767	74,719
Comprehensive income:
Net income	$ 20,190	$ 9,156
Other comprehensive income:
Unrealized (loss) gain on available-for-sale marketable securities, net of taxes	(533)	1,416
Total comprehensive income	$ 19,657	$ 10,572