Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Revenues	$ 1,866,131	$ 1,562,662	$ 1,436,849
Operating costs and expenses:
Costs of services provided	1,612,510	1,339,492	1,236,108
Selling, general and administrative	126,732	105,417	111,689
Other income:
Investment and interest	6,076	2,634	712
Income before income taxes	132,965	120,387	89,764
Income taxes	44,739	42,991	31,740
Net income	$ 88,226	$ 77,396	$ 58,024
Per share data:
Basic earnings per common share (in dollars per share)	$ 1.20	$ 1.06	$ 0.81
Diluted earnings per common share (in dollars per share)	$ 1.19	$ 1.05	$ 0.80
Weighted average number of common shares outstanding:
Basic (in shares)	73,355	72,754	71,826
Diluted (in shares)	74,348	73,474	72,512
Comprehensive income:
Net income	$ 88,226	$ 77,396	$ 58,024
Other comprehensive income:
Unrealized gain (loss) on available-for-sale marketable securities, net of taxes	1,156	(862)	518
Total comprehensive income	$ 89,382	$ 76,534	$ 58,542