Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Revenues	$ 1,840,778	$ 2,002,601	$ 1,861,206
Operating costs and expenses:
Costs of services provided	1,612,877	1,768,162	1,610,590
Selling, general and administrative	150,022	136,603	126,732
Other income (expense):
Investment and other income, net	10,676	5,168	10,444
Interest expense	(3,459)	(3,094)	(1,363)
Income before income taxes	85,096	99,910	132,965
Income taxes	20,515	16,386	44,739
Net income	$ 64,581	$ 83,524	$ 88,226
Per share data:
Basic earnings per common share (in dollars per share)	$ 0.87	$ 1.13	$ 1.20
Diluted earnings per common share (in dollars per share)	$ 0.87	$ 1.12	$ 1.19
Weighted average number of common shares outstanding:
Basic (in shares)	74,362	74,002	73,355
Diluted (in shares)	74,590	74,612	74,348
Comprehensive income:
Net income	$ 64,581	$ 83,524	$ 88,226
Other comprehensive income:
Unrealized gain (loss) on available-for-sale marketable securities, net of taxes	2,761	(679)	1,156
Total comprehensive income	$ 67,342	$ 82,845	$ 89,382