Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Net service revenues	$ 5,266,789	$ 5,250,399	$ 5,094,933
Direct costs of services, consisting of payroll, payroll taxes, benefit costs and reimbursable expenses	3,102,977	3,089,723	2,980,462
Gross margin	2,163,812	2,160,676	2,114,471
Selling, general and administrative expenses	1,646,532	1,606,217	1,533,799
Amortization of intangible assets	1,563	1,237	192
Interest income, net	(1,799)	(888)	(550)
Income before income taxes	517,516	554,110	581,030
Provision for income taxes	226,932	210,721	223,234
Net income	$ 290,584	$ 343,389	$ 357,796
Net income per share :
Basic (usd per share)	$ 2.34	$ 2.68	$ 2.72
Diluted (usd per share)	$ 2.33	$ 2.67	$ 2.69
Shares:
Basic (in shares)	124,152	127,991	131,749
Diluted (in shares)	124,892	128,766	132,930
Cash dividends declared per share (usd per share)	$ 0.96	$ 0.88	$ 0.8