Consolidated Statements Of Income (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Revenues	$ 2,869,982	$ 2,604,038	$ 2,020,334
Cost of services	2,453,679	2,208,631	1,668,763
General and administrative expense	188,949	180,267	172,492
Operating income	227,354	215,140	179,079
Interest expense	(15,791)	(12,567)	(1,141)
Interest income	332	361	215
Other income (expense), net	3,607	(483)	355
INCOME FROM OPERATIONS BEFORE INCOME TAXES	215,502	202,451	178,508
Provision for income taxes	(82,196)	(77,355)	(66,744)
Net income	$ 133,306	$ 125,096	$ 111,764
Class A Common Stock [Member]
BASIC EARNINGS PER SHARE:
Basic earnings per share	$ 3.64	$ 3.45	$ 3.13
Weighted average common shares outstanding	23,415	22,847	21,980
DILUTED EARNINGS PER SHARE:
Diluted earnings per share	$ 3.63	$ 3.43	$ 3.11
Weighted average common shares outstanding	23,530	23,054	22,278
Class B Common Stock [Member]
BASIC EARNINGS PER SHARE:
Basic earnings per share	$ 3.64	$ 3.45	$ 3.13
Weighted average common shares outstanding	13,233	13,367	13,707
DILUTED EARNINGS PER SHARE:
Diluted earnings per share	$ 3.63	$ 3.43	$ 3.11
Weighted average common shares outstanding	13,233	13,367	13,707