Condensed Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Income Statement [Abstract]
Net service revenues	$ 1,308,428	$ 1,344,160	$ 2,595,798	$ 2,646,785
Direct costs of services, consisting of payroll, payroll taxes, benefit costs and reimbursable expenses	769,990	787,167	1,531,532	1,557,820
Gross margin	538,438	556,993	1,064,266	1,088,965
Selling, general and administrative expenses	407,721	407,496	807,970	805,570
Amortization of intangible assets	365	314	666	602
Interest income, net	(355)	(231)	(578)	(412)
Income before income taxes	130,707	149,414	256,208	283,205
Provision for income taxes	50,391	57,798	97,371	108,173
Net income	$ 80,316	$ 91,616	$ 158,837	$ 175,032
Net income per share:
Basic (in usd per share)	$ 0.64	$ 0.71	$ 1.27	$ 1.36
Diluted (in usd per share)	$ 0.64	$ 0.71	$ 1.26	$ 1.35
Shares:
Basic (in shares)	124,617	128,586	125,074	128,933
Diluted (in shares)	125,104	129,329	125,758	129,733
Cash dividends declared per share (in usd per share)	$ 0.24	$ 0.22	$ 0.48	$ 0.44