Quarterly Financial Data (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Net service revenues	$ 1,221,450	$ 1,224,308	$ 1,164,914	$ 1,084,342	$ 1,083,864	$ 1,075,119	$ 1,063,228	$ 1,023,684	$ 4,695,014	$ 4,245,895	$ 4,111,213
Gross margin	500,757	505,220	478,444	438,495	443,216	437,478	432,108	410,290	1,922,916	1,723,092	1,649,060
Income before income taxes	133,321	138,361	123,653	102,014	103,894	103,713	100,596	89,376	497,349	397,579	344,245
Net income	84,053	85,184	75,140	61,551	66,885	66,358	63,089	55,863	305,928	252,195	209,942
Net income available to common stockholders - diluted	$ 84,053	$ 85,184	$ 75,140	$ 61,551	$ 66,885	$ 66,358	$ 63,088	$ 55,861	$ 305,928	$ 252,192	$ 208,867
Basic net income per share	$ 0.63	$ 0.64	$ 0.56	$ 0.45	$ 0.49	$ 0.49	$ 0.46	$ 0.41	$ 2.28	$ 1.85	$ 1.51
Diluted net income per share	$ 0.62	$ 0.63	$ 0.55	$ 0.45	$ 0.49	$ 0.48	$ 0.46	$ 0.40	$ 2.26	$ 1.83	$ 1.50