Business Segments - Reconciliation of Revenue and Operating Income by Reportable Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Segment Reporting Information [Line Items]
Net service revenues	$ 1,272,058	$ 1,164,914	$ 2,477,621	$ 2,249,256
Operating income	149,147	123,653	277,249	225,762
Amortization of intangible assets	0	224	0	557
Interest income, net	(88)	(224)	(160)	(462)
Income before income taxes	149,235	123,653	277,409	225,667
Temporary and consultant staffing
Segment Reporting Information [Line Items]
Net service revenues	979,602	911,038	1,923,120	1,768,608
Operating income	104,612	88,756	197,413	166,241
Permanent placement staffing
Segment Reporting Information [Line Items]
Net service revenues	110,583	102,827	208,996	195,452
Operating income	24,052	22,448	43,083	39,723
Risk consulting and internal audit services
Segment Reporting Information [Line Items]
Net service revenues	181,873	151,049	345,505	285,196
Operating income	$ 20,483	$ 12,449	$ 36,753	$ 19,798