Condensed Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Statement [Abstract]
Net service revenues	$ 1,516,385	$ 1,457,054	$ 2,984,915	$ 2,852,387
Direct costs of services, consisting of payroll, payroll taxes, benefit costs and reimbursable expenses	878,844	849,936	1,739,786	1,672,903
Gross margin	637,541	607,118	1,245,129	1,179,484
Selling, general and administrative expenses	478,139	457,607	939,498	895,606
Amortization of intangible assets	341	442	683	905
Interest income, net	(1,042)	(1,006)	(2,538)	(1,741)
Income before income taxes	160,103	150,075	307,486	284,714
Provision for income taxes	45,491	40,760	83,076	79,232
Net income	$ 114,612	$ 109,315	$ 224,410	$ 205,482
Net income per share:
Basic (in usd per share)	$ 0.98	$ 0.90	$ 1.92	$ 1.69
Diluted (in usd per share)	$ 0.98	$ 0.89	$ 1.91	$ 1.68
Shares:
Basic (in shares)	116,381	121,307	116,722	121,619
Diluted (in shares)	116,988	122,268	117,475	122,576
Cash dividends declared per share (in usd per share)	$ 0.31	$ 0.28	$ 0.62	$ 0.56