Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Net service revenues	$ 5,800,271	$ 5,266,789	$ 5,250,399
Direct costs of services, consisting of payroll, payroll taxes, benefit costs and reimbursable expenses	3,390,257	3,102,977	3,089,723
Gross margin	2,410,014	2,163,812	2,160,676
Selling, general and administrative expenses	1,821,089	1,646,532	1,606,217
Amortization of intangible assets	1,705	1,563	1,237
Interest income, net	(4,382)	(1,799)	(888)
Income before income taxes	591,602	517,516	554,110
Provision for income taxes	157,314	226,932	210,721
Net income	$ 434,288	$ 290,584	$ 343,389
Net income per share :
Basic (usd per share)	$ 3.60	$ 2.34	$ 2.68
Diluted (usd per share)	$ 3.57	$ 2.33	$ 2.67
Shares:
Basic (in shares)	120,513	124,152	127,991
Diluted (in shares)	121,602	124,892	128,766
Cash dividends declared per share (usd per share)	$ 1.12	$ 0.96	$ 0.88