Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
Net service revenues	$ 5,250,399	$ 5,094,933	$ 4,695,014
Direct costs of services, consisting of payroll, payroll taxes, benefit costs and reimbursable expenses	3,089,723	2,980,462	2,772,098
Gross margin	2,160,676	2,114,471	1,922,916
Selling, general and administrative expenses	1,606,217	1,533,799	1,425,734
Amortization of intangible assets	1,237	192	557
Interest income, net	(888)	(550)	(724)
Income before income taxes	554,110	581,030	497,349
Provision for income taxes	210,721	223,234	191,421
Net income	$ 343,389	$ 357,796	$ 305,928
Net income per share:
Basic (usd per share)	$ 2.68	$ 2.72	$ 2.28
Diluted (usd per share)	$ 2.67	$ 2.69	$ 2.26
Shares:
Basic (in shares)	127,991	131,749	134,358
Diluted (in shares)	128,766	132,930	135,541
Cash dividends declared per share (usd per share)	$ 0.88	$ 0.8	$ 0.72