Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Net service revenues	$ 1,304,594	$ 1,312,718	$ 1,272,058	$ 1,205,563	$ 1,221,450	$ 1,224,308	$ 1,164,914	$ 1,084,342	$ 5,094,933	$ 4,695,014	$ 4,245,895
Gross margin	540,081	549,801	530,502	494,087	500,757	505,220	478,444	438,495	2,114,471	1,922,916	1,723,092
Income before income taxes	144,315	159,306	149,235	128,174	133,321	138,361	123,653	102,014	581,030	497,349	397,579
Net income	$ 93,443	$ 96,725	$ 89,706	$ 77,922	$ 84,053	$ 85,184	$ 75,140	$ 61,551	$ 357,796	$ 305,928	$ 252,195
Basic net income per share (in USD per share)	$ 0.71	$ 0.74	$ 0.68	$ 0.59	$ 0.63	$ 0.64	$ 0.56	$ 0.45	$ 2.72	$ 2.28	$ 1.85
Diluted net income per share (in USD per share)	$ 0.71	$ 0.73	$ 0.67	$ 0.58	$ 0.62	$ 0.63	$ 0.55	$ 0.45	$ 2.69	$ 2.26	$ 1.83