Quarterly Financial Data (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended									12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012		Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Quarterly Financial Information [Line Items]
Net service revenues	$ 1,034,213	$ 1,033,173	$ 1,028,383		$ 1,015,444	$ 973,473	$ 984,668	$ 937,966	$ 880,869	$ 4,111,213	$ 3,776,976	$ 3,175,093
Gross margin	416,425	415,249	415,303		402,083	387,322	391,903	372,288	338,089	1,649,060	1,489,602	1,194,033
Income before income taxes	97,242	94,467	72,272	[1]	80,264	72,161	72,195	61,284	44,576	344,245	250,216	115,168
Net income	58,619	57,660	45,329	[1]	48,334	42,624	44,168	36,425	26,705	209,942	149,922	66,069
Net income available to common stockholders-diluted	$ 58,322	$ 57,383	$ 45,101		$ 48,070	$ 42,037	$ 43,556	$ 35,890	$ 26,293	$ 208,867	$ 147,772	$ 63,729
Basic net income per share	$ 0.43	$ 0.42	$ 0.33		$ 0.34	$ 0.30	$ 0.31	$ 0.25	$ 0.18	$ 1.51	$ 1.05	$ 0.45
Diluted net income per share	$ 0.42	$ 0.41	$ 0.32	[1]	$ 0.34	$ 0.30	$ 0.31	$ 0.25	$ 0.18	$ 1.50	$ 1.04	$ 0.44

[1] The decrease in second quarter 2012 income before income taxes, net income, and net income per share was primarily due to a charge related to a litigation settlement disclosed in the Company's July 5, 2012, Form 8-K. The settlement resulted in a non-recurring $19.0 million pre-tax charge to selling, general and administrative expenses and reduced second quarter net income and net income per share by $11.4 million and $0.08 per share, respectively.