Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Revenues	$ 2,008,821	$ 1,866,131	$ 1,562,662
Operating costs and expenses:
Costs of services provided	1,771,981	1,612,510	1,339,492
Selling, general and administrative	136,603	126,732	105,417
Other (expense) income:
Investment and interest	(327)	6,076	2,634
Income before income taxes	99,910	132,965	120,387
Income taxes	16,386	44,739	42,991
Net income	$ 83,524	$ 88,226	$ 77,396
Per share data:
Basic earnings per common share (in dollars per share)	$ 1.13	$ 1.20	$ 1.06
Diluted earnings per common share (in dollars per share)	$ 1.12	$ 1.19	$ 1.05
Weighted average number of common shares outstanding:
Basic (in shares)	74,002	73,355	72,754
Diluted (in shares)	74,612	74,348	73,474
Comprehensive income:
Net income	$ 83,524	$ 88,226	$ 77,396
Other comprehensive income:
Unrealized (loss) gain on available-for-sale marketable securities, net of taxes	(679)	1,156	(862)
Total comprehensive income	$ 82,845	$ 89,382	$ 76,534