Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Net service revenues	$ 1,346,282	$ 1,324,709	$ 1,308,428	$ 1,287,370	$ 1,265,073	$ 1,338,541	$ 1,344,160	$ 1,302,625	$ 5,266,789	$ 5,250,399	$ 5,094,933
Gross margin	553,146	546,400	538,438	525,828	519,202	552,509	556,993	531,972	2,163,812	2,160,676	2,114,471
Income before income taxes	129,038	132,270	130,707	125,501	124,581	146,324	149,414	133,791	517,516	554,110	581,030
Net income	$ 47,047	$ 84,700	$ 80,316	$ 78,521	$ 77,788	$ 90,569	$ 91,616	$ 83,416	$ 290,584	$ 343,389	$ 357,796
Basic net income per share (in USD per share)	$ 0.38	$ 0.69	$ 0.64	$ 0.63	$ 0.61	$ 0.71	$ 0.71	$ 0.65	$ 2.34	$ 2.68	$ 2.72
Diluted net income per share (in USD per share)	$ 0.38	$ 0.68	$ 0.64	$ 0.62	$ 0.61	$ 0.71	$ 0.71	$ 0.64	$ 2.33	$ 2.67	$ 2.69