Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Income Statement [Abstract]
Revenues	$ 452,029	$ 462,101	$ 901,179	$ 938,212
Operating costs and expenses:
Costs of services provided	387,517	400,485	774,673	827,750
Selling, general and administrative expense	41,465	38,609	71,482	79,710
Other income (expense):
Investment and other income, net	7,745	1,393	2,918	6,596
Interest expense	(380)	(783)	(748)	(1,839)
Income before income taxes	30,412	23,617	57,194	35,509
Income tax provision	7,311	5,431	13,903	8,167
Net income	$ 23,101	$ 18,186	$ 43,291	$ 27,342
Per share data:
Basic earnings per common share (in dollars per share)	$ 0.31	$ 0.24	$ 0.58	$ 0.37
Diluted earnings per common share (in dollars per share)	$ 0.31	$ 0.24	$ 0.58	$ 0.37
Weighted average number of common shares outstanding:
Basic (in shares)	74,695	74,352	74,676	74,327
Diluted (in shares)	74,761	74,619	74,764	74,669
Comprehensive income:
Net income	$ 23,101	$ 18,186	$ 43,291	$ 27,342
Other comprehensive income:
Unrealized gain on available-for-sale marketable securities, net of taxes	1,656	791	1,123	2,207
Total comprehensive income	$ 24,757	$ 18,977	$ 44,414	$ 29,549