Condensed Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Income Statement [Abstract]
Net service revenues	$ 1,272,058	$ 1,164,914	$ 2,477,621	$ 2,249,256
Direct costs of services, consisting of payroll, payroll taxes, benefit costs and reimbursable expenses	741,556	686,470	1,453,032	1,332,317
Gross margin	530,502	478,444	1,024,589	916,939
Selling, general and administrative expenses	381,355	354,791	747,340	691,177
Amortization of intangible assets	0	224	0	557
Interest income, net	(88)	(224)	(160)	(462)
Income before income taxes	149,235	123,653	277,409	225,667
Provision for income taxes	59,529	48,513	109,781	88,976
Net income	$ 89,706	$ 75,140	$ 167,628	$ 136,691
Net income per share:
Basic (in usd per share)	$ 0.68	$ 0.56	$ 1.26	$ 1.01
Diluted (in usd per share)	$ 0.67	$ 0.55	$ 1.25	$ 1.01
Shares:
Basic (in shares)	132,499	134,699	132,786	135,014
Diluted (in shares)	133,553	135,708	133,918	135,932
Cash dividends declared per share	$ 0.20	$ 0.18	$ 0.40	$ 0.36