Condensed Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Net service revenues	$ 1,457,054	$ 1,308,428	$ 2,852,387	$ 2,595,798
Direct costs of services, consisting of payroll, payroll taxes, benefit costs and reimbursable expenses	849,936	769,990	1,672,903	1,531,532
Gross margin	607,118	538,438	1,179,484	1,064,266
Selling, general and administrative expenses	457,607	407,721	895,606	807,970
Amortization of intangible assets	442	365	905	666
Interest income, net	(1,006)	(355)	(1,741)	(578)
Income before income taxes	150,075	130,707	284,714	256,208
Provision for income taxes	40,760	50,391	79,232	97,371
Net income	$ 109,315	$ 80,316	$ 205,482	$ 158,837
Net income per share:
Basic (in usd per share)	$ 0.90	$ 0.64	$ 1.69	$ 1.27
Diluted (in usd per share)	$ 0.89	$ 0.64	$ 1.68	$ 1.26
Shares:
Basic (in shares)	121,307	124,617	121,619	125,074
Diluted (in shares)	122,268	125,104	122,576	125,758
Cash dividends declared per share (in usd per share)	$ 0.28	$ 0.24	$ 0.56	$ 0.48