Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Statement [Abstract]
Revenues	$ 476,111	$ 500,562
Operating costs and expenses:
Costs of services provided	427,265	469,252
Selling, general and administrative expense	41,101	33,777
Other income (expense):
Investment and other income, net	5,203	1,796
Interest expense	(1,056)	(724)
Income (loss) before income taxes	11,892	(1,395)
Income tax provision (benefit)	2,736	(1,467)
Net income	$ 9,156	$ 72
Per share data:
Basic earnings per common share (in dollars per share)	$ 0.12	$ 0.00
Diluted earnings per common share (in dollars per share)	$ 0.12	$ 0.00
Weighted average number of common shares outstanding:
Basic (in shares)	74,301	73,913
Diluted (in shares)	74,719	74,725
Comprehensive income:
Net income	$ 9,156	$ 72
Other comprehensive income:
Unrealized gain (loss) on available-for-sale marketable securities, net of taxes	1,416	(1,138)
Total comprehensive income (loss)	$ 10,572	$ (1,066)