Quarterly Financial Data (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Financial Information Disclosure [Abstract]
Net service revenues	$ 1,083,864	$ 1,075,119	$ 1,063,228	$ 1,023,684	$ 1,034,213	$ 1,033,173	$ 1,028,383	$ 1,015,444	$ 4,245,895	$ 4,111,213	$ 3,776,976
Gross margin	443,216	437,478	432,108	410,290	416,425	415,249	415,303	402,083	1,723,092	1,649,060	1,489,602
Income before income taxes	103,894	103,713	100,596	89,376	97,242	94,467	72,272	80,264	397,579	344,245	250,216
Net income	66,885	66,358	63,089	55,863	58,619	57,660	45,329	48,334	252,195	209,942	149,922
Net income available to common stockholders - diluted	$ 66,885	$ 66,358	$ 63,088	$ 55,861	$ 58,322	$ 57,383	$ 45,101	$ 48,070	$ 252,192	$ 208,867	$ 147,772
Basic net income per share	$ 0.49	$ 0.49	$ 0.46	$ 0.41	$ 0.43	$ 0.42	$ 0.33	$ 0.34	$ 1.85	$ 1.51	$ 1.05
Diluted net income per share	$ 0.49	$ 0.48	$ 0.46	$ 0.40	$ 0.42	$ 0.41	$ 0.32	$ 0.34	$ 1.83	$ 1.50	$ 1.04