Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Net service revenues	$ 1,265,073	$ 1,338,541	$ 1,344,160	$ 1,302,625	$ 1,304,594	$ 1,312,718	$ 1,272,058	$ 1,205,563	$ 5,250,399	$ 5,094,933	$ 4,695,014
Gross margin	519,202	552,509	556,993	531,972	540,081	549,801	530,502	494,087	2,160,676	2,114,471	1,922,916
Income before income taxes	124,581	146,324	149,414	133,791	144,315	159,306	149,235	128,174	554,110	581,030	497,349
Net income	$ 77,788	$ 90,569	$ 91,616	$ 83,416	$ 93,443	$ 96,725	$ 89,706	$ 77,922	$ 343,389	$ 357,796	$ 305,928
Basic net income per share (in USD per share)	$ 0.61	$ 0.71	$ 0.71	$ 0.65	$ 0.71	$ 0.74	$ 0.68	$ 0.59	$ 2.68	$ 2.72	$ 2.28
Diluted net income per share (in USD per share)	$ 0.61	$ 0.71	$ 0.71	$ 0.64	$ 0.71	$ 0.73	$ 0.67	$ 0.58	$ 2.67	$ 2.69	$ 2.26