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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Net service revenues	$ 1,265,073	$ 1,338,541	$ 1,344,160	$ 1,302,625	$ 1,304,594	$ 1,312,718	$ 1,272,058	$ 1,205,563	$ 5,250,399	$ 5,094,933	$ 4,695,014
Amortization of intangible assets									1,237	192	557
Interest income, net									(888)	(550)	(724)
Income before income taxes	$ 124,581	$ 146,324	$ 149,414	$ 133,791	$ 144,315	$ 159,306	$ 149,235	$ 128,174	554,110	581,030	497,349
Operating segments
Segment Reporting Information [Line Items]
Net service revenues									5,250,399	5,094,933	4,695,014
Operating income									554,459	580,672	497,182
Operating segments | Temporary and consultant staffing
Segment Reporting Information [Line Items]
Net service revenues									4,026,777	3,930,843	3,676,281
Operating income									393,704	399,808	358,533
Operating segments | Permanent placement staffing
Segment Reporting Information [Line Items]
Net service revenues									419,314	421,411	394,515
Operating income									80,001	85,019	78,333
Operating segments | Risk consulting and internal audit services
Segment Reporting Information [Line Items]
Net service revenues									804,308	742,679	624,218
Operating income									80,754	95,845	60,316
Segment reconciling items
Segment Reporting Information [Line Items]
Amortization of intangible assets									1,237	192	557
Interest income, net									$ (888)	$ (550)	$ (724)