Consolidated Statements Of Comprehensive Income (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income Statement [Abstract]
Revenues	$ 1,077,435	$ 889,065	$ 773,956
Operating costs and expenses:
Costs of services provided	930,814	766,958	665,149
Selling, general and administrative	79,277	65,306	57,310
Other income:
Investment and interest	2,920	1,011	2,622
Income before income taxes	70,264	57,812	54,119
Income taxes	26,050	19,656	19,678
Net income	44,214	38,156	34,441
Per share data:
Basic earnings per common share (dollars per share)	$ 0.65	$ 0.57	$ 0.52
Diluted earnings per common share (dollars per share)	$ 0.65	$ 0.56	$ 0.51
Cash dividends per common share (dollars per share)	$ 0.65	$ 0.63	$ 0.60
Weighted average number of common shares outstanding:
Basic (shares)	67,511	66,637	65,917
Diluted (shares)	68,485	67,585	67,008
Comprehensive income:
Net income	44,214	38,156	34,441
Other comprehensive income:
Unrealized gain/(loss) on available for sale marketable securities, net of taxes	(216)	421	(78)
Total Comprehensive Income	$ 43,998	$ 38,577	$ 34,363