Business Segments - Reconciliation of Revenue and Operating Income by Reportable Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Segment Reporting Information [Line Items]
Net service revenues	$ 1,302,625	$ 1,205,563
Operating income	133,898	128,102
Amortization of intangible assets	288	0
Interest income, net	(181)	(72)
Income before income taxes	133,791	128,174
Temporary and consultant staffing
Segment Reporting Information [Line Items]
Net service revenues	1,009,165	943,518
Operating income	97,883	92,801
Permanent placement staffing
Segment Reporting Information [Line Items]
Net service revenues	106,289	98,413
Operating income	21,502	19,031
Risk consulting and internal audit services
Segment Reporting Information [Line Items]
Net service revenues	187,171	163,632
Operating income	14,513	16,270
Segment Reconciling Items
Segment Reporting Information [Line Items]
Amortization of intangible assets	288	0
Interest income, net	$ (181)	$ (72)