Business Segments - Reconciliation of Revenue and Operating Income by Reportable Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Segment Reporting Information [Line Items]
Net service revenues	$ 1,312,718	$ 1,224,308	$ 3,790,339	$ 3,473,564
Operating income	159,066	138,253	436,315	364,015
Amortization of intangible assets	0	0	0	557
Interest income, net	(240)	(108)	(400)	(570)
Income before income taxes	159,306	138,361	436,715	364,028
Temporary and consultant staffing
Segment Reporting Information [Line Items]
Net service revenues	999,822	954,225	2,922,942	2,722,833
Operating income	102,991	97,660	300,404	263,901
Permanent placement staffing
Segment Reporting Information [Line Items]
Net service revenues	110,748	102,345	319,744	297,797
Operating income	24,377	21,380	67,460	61,103
Risk consulting and internal audit services
Segment Reporting Information [Line Items]
Net service revenues	202,148	167,738	547,653	452,934
Operating income	$ 31,698	$ 19,213	$ 68,451	$ 39,011