Business Segments - Reconciliation of Revenue and Operating Income by Reportable Segment to Consolidated Results (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Net service revenues	$ 1,304,594	$ 1,312,718	$ 1,272,058	$ 1,205,563	$ 1,221,450	$ 1,224,308	$ 1,164,914	$ 1,084,342	$ 5,094,933	$ 4,695,014	$ 4,245,895
Amortization of intangible assets									192	557	1,700
Interest income, net									(550)	(724)	(1,002)
Income before income taxes	$ 144,315	$ 159,306	$ 149,235	$ 128,174	$ 133,321	$ 138,361	$ 123,653	$ 102,014	581,030	497,349	397,579
Operating segments
Segment Reporting Information [Line Items]
Net service revenues									5,094,933	4,695,014	4,245,895
Operating income									580,672	497,182	398,277
Operating segments | Temporary and consultant staffing
Segment Reporting Information [Line Items]
Net service revenues									3,930,843	3,676,281	3,369,840
Operating income									399,808	358,533	301,185
Operating segments | Permanent placement staffing
Segment Reporting Information [Line Items]
Net service revenues									421,411	394,515	347,715
Operating income									85,019	78,333	54,390
Operating segments | Risk consulting and internal audit services
Segment Reporting Information [Line Items]
Net service revenues									742,679	624,218	528,340
Operating income									95,845	60,316	42,702
Segment reconciling items
Segment Reporting Information [Line Items]
Amortization of intangible assets									192	557	1,700
Interest income, net									$ (550)	$ (724)	$ (1,002)