Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Net service revenues	$ 1,481,658	$ 1,466,226	$ 1,457,054	$ 1,395,333	$ 1,346,282	$ 1,324,709	$ 1,308,428	$ 1,287,370	$ 5,800,271	$ 5,266,789	$ 5,250,399
Gross margin	620,062	610,468	607,118	572,366	553,146	546,400	538,438	525,828	2,410,014	2,163,812	2,160,676
Income before income taxes	154,983	151,905	150,075	134,639	129,038	132,270	130,707	125,501	591,602	517,516	554,110
Net income	$ 113,564	$ 115,242	$ 109,315	$ 96,167	$ 47,047	$ 84,700	$ 80,316	$ 78,521	$ 434,288	$ 290,584	$ 343,389
Basic net income per share (in USD per share)	$ 0.96	$ 0.96	$ 0.90	$ 0.79	$ 0.38	$ 0.69	$ 0.64	$ 0.63	$ 3.60	$ 2.34	$ 2.68
Diluted net income per share (in USD per share)	$ 0.95	$ 0.95	$ 0.89	$ 0.78	$ 0.38	$ 0.68	$ 0.64	$ 0.62	$ 3.57	$ 2.33	$ 2.67