Condensed Consolidated Statements of Operations (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Income Statement [Abstract]
Net service revenues	$ 1,164,914	$ 1,063,228	$ 2,249,256	$ 2,086,912
Direct costs of services, consisting of payroll, payroll taxes, insurance costs and reimbursable expenses	686,470	631,120	1,332,317	1,244,514
Gross margin	478,444	432,108	916,939	842,398
Selling, general and administrative expenses	354,791	331,314	691,177	652,099
Amortization of intangible assets	224	434	557	867
Interest income, net	(224)	(236)	(462)	(540)
Income before income taxes	123,653	100,596	225,667	189,972
Provision for income taxes	48,513	37,507	88,976	71,020
Net income	75,140	63,089	136,691	118,952
Net income available to common stockholders-diluted	$ 75,140	$ 63,088	$ 136,691	$ 118,948
Net income per share (Note I):
Basic	$ 0.56	$ 0.46	$ 1.01	$ 0.87
Diluted	$ 0.55	$ 0.46	$ 1.01	$ 0.86
Shares:
Basic	134,699	136,385	135,014	136,845
Diluted	135,708	137,659	135,932	138,024
Cash dividends declared per share	$ 0.18	$ 0.16	$ 0.36	$ 0.32