Condensed Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Income Statement [Abstract]
Net service revenues	$ 1,338,541	$ 1,312,718	$ 3,985,326	$ 3,790,339
Direct costs of services, consisting of payroll, payroll taxes, benefit costs and reimbursable expenses	786,032	762,917	2,343,852	2,215,949
Gross margin	552,509	549,801	1,641,474	1,574,390
Selling, general and administrative expenses	406,142	390,735	1,211,712	1,138,075
Amortization of intangible assets	326	0	928	0
Interest income, net	(283)	(240)	(695)	(400)
Income before income taxes	146,324	159,306	429,529	436,715
Provision for income taxes	55,755	62,581	163,928	172,362
Net income	$ 90,569	$ 96,725	$ 265,601	$ 264,353
Net income per share:
Basic (in usd per share)	$ 0.71	$ 0.74	$ 2.07	$ 2.00
Diluted (in usd per share)	$ 0.71	$ 0.73	$ 2.06	$ 1.98
Shares:
Basic (in shares)	127,615	131,285	128,491	132,280
Diluted (in shares)	128,191	132,488	129,216	133,436
Cash dividends declared per share (in usd per share)	$ 0.22	$ 0.20	$ 0.66	$ 0.60