Quarterly Financial Data (Schedule Of Quarterly Financial Information) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Quarterly Financial Data [Abstract]
Net service revenues	$ 973,473	$ 984,668	$ 937,966	$ 880,869	$ 851,572	$ 817,258	$ 769,090	$ 737,173	$ 3,776,976	$ 3,175,093	$ 3,036,547
Gross margin	387,322	391,903	372,288	338,089	326,907	309,140	289,858	268,128	1,489,602	1,194,033	1,103,679
Income before income taxes	72,161	72,195	61,284	44,576	43,248	37,268	22,387	12,265	250,216	115,168	66,763
Net income	42,624	44,168	36,425	26,705	24,790	20,623	12,181	8,475	149,922	66,069	37,263
Net income available to common stockholders - diluted	$ 42,037	$ 43,556	$ 35,890	$ 26,293	$ 24,144	$ 20,099	$ 11,660	$ 7,626	$ 147,772	$ 63,729	$ 35,067
Basic net income per share	$ 0.30	$ 0.31	$ 0.25	$ 0.18	$ 0.17	$ 0.14	$ 0.08	$ 0.05	$ 1.05	$ 0.45	$ 0.24
Diluted net income per share	$ 0.30	$ 0.31	$ 0.25	$ 0.18	$ 0.17	$ 0.14	$ 0.08	$ 0.05	$ 1.04	$ 0.44	$ 0.24