Condensed Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Statement [Abstract]
Net service revenues	$ 1,468,530	$ 1,395,333
Direct costs of services, consisting of payroll, payroll taxes, benefit costs and reimbursable expenses	860,942	822,967
Gross margin	607,588	572,366
Selling, general and administrative expenses	461,359	437,999
Amortization of intangible assets	342	463
Interest income, net	(1,496)	(735)
Income before income taxes	147,383	134,639
Provision for income taxes	37,585	38,472
Net income	$ 109,798	$ 96,167
Net income per share:
Basic (in usd per share)	$ 0.94	$ 0.79
Diluted (in usd per share)	$ 0.93	$ 0.78
Shares:
Basic (in shares)	117,068	121,934
Diluted (in shares)	117,966	122,887
Cash dividends declared per share (in usd per share)	$ 0.31	$ 0.28