Condensed Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Net service revenues	$ 1,466,226	$ 1,324,709	$ 4,318,613	$ 3,920,507
Direct costs of services, consisting of payroll, payroll taxes, benefit costs and reimbursable expenses	855,758	778,309	2,528,661	2,309,841
Gross margin	610,468	546,400	1,789,952	1,610,666
Selling, general and administrative expenses	459,330	414,179	1,354,936	1,222,149
Amortization of intangible assets	429	481	1,334	1,147
Interest income, net	(1,196)	(530)	(2,937)	(1,108)
Income before income taxes	151,905	132,270	436,619	388,478
Provision for income taxes	36,663	47,570	115,895	144,941
Net income	$ 115,242	$ 84,700	$ 320,724	$ 243,537
Net income per share:
Basic (in usd per share)	$ 0.96	$ 0.69	$ 2.65	$ 1.96
Diluted (in usd per share)	$ 0.95	$ 0.68	$ 2.62	$ 1.94
Shares:
Basic (in shares)	120,115	123,569	121,112	124,567
Diluted (in shares)	121,443	124,200	122,193	125,233
Cash dividends declared per share (in usd per share)	$ 0.28	$ 0.24	$ 0.84	$ 0.72