Condensed Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Income Statement [Abstract]
Net service revenues	$ 1,344,160	$ 1,272,058	$ 2,646,785	$ 2,477,621
Direct costs of services, consisting of payroll, payroll taxes, benefit costs and reimbursable expenses	787,167	741,556	1,557,820	1,453,032
Gross margin	556,993	530,502	1,088,965	1,024,589
Selling, general and administrative expenses	407,496	381,355	805,570	747,340
Amortization of intangible assets	314	0	602	0
Interest income, net	(231)	(88)	(412)	(160)
Income before income taxes	149,414	149,235	283,205	277,409
Provision for income taxes	57,798	59,529	108,173	109,781
Net income	$ 91,616	$ 89,706	$ 175,032	$ 167,628
Net income per share:
Basic (in usd per share)	$ 0.71	$ 0.68	$ 1.36	$ 1.26
Diluted (in usd per share)	$ 0.71	$ 0.67	$ 1.35	$ 1.25
Shares:
Basic (in shares)	128,586	132,499	128,933	132,786
Diluted (in shares)	129,329	133,553	129,733	133,918
Cash dividends declared per share (in usd per share)	$ 0.22	$ 0.20	$ 0.44	$ 0.40