Business Segments Reconciliation of Revenue and Operating Income by Reportable Segment (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Segment Reporting Information [Line Items]
Net service revenues	$ 1,224,308	$ 1,075,119	$ 3,473,564	$ 3,162,031
Operating income	138,253	103,913	364,015	294,212
Amortization of intangible assets	0	433	557	1,300
Interest income, net	(108)	(233)	(570)	(773)
Income before income taxes	138,361	103,713	364,028	293,685
Temporary and consultant staffing
Segment Reporting Information [Line Items]
Net service revenues	954,225	847,886	2,722,833	2,513,377
Operating income	97,660	76,532	263,901	226,293
Permanent placement staffing
Segment Reporting Information [Line Items]
Net service revenues	102,345	88,222	297,797	261,946
Operating income	21,380	13,691	61,103	41,753
Risk consulting and internal audit services
Segment Reporting Information [Line Items]
Net service revenues	167,738	139,011	452,934	386,708
Operating income	$ 19,213	$ 13,690	$ 39,011	$ 26,166