Condensed Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Statement [Abstract]
Net service revenues	$ 1,395,333	$ 1,287,370
Direct costs of services, consisting of payroll, payroll taxes, benefit costs and reimbursable expenses	822,967	761,542
Gross margin	572,366	525,828
Selling, general and administrative expenses	437,999	400,249
Amortization of intangible assets	463	301
Interest income, net	(735)	(223)
Income before income taxes	134,639	125,501
Provision for income taxes	38,472	46,980
Net income	$ 96,167	$ 78,521
Net income per share:
Basic (in usd per share)	$ 0.79	$ 0.63
Diluted (in usd per share)	$ 0.78	$ 0.62
Shares:
Basic (in shares)	121,934	125,537
Diluted (in shares)	122,887	126,418
Cash dividends declared per share (in usd per share)	$ 0.28	$ 0.24