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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Net service revenues	$ 1,346,282	$ 1,324,709	$ 1,308,428	$ 1,287,370	$ 1,265,073	$ 1,338,541	$ 1,344,160	$ 1,302,625	$ 5,266,789	$ 5,250,399	$ 5,094,933
Amortization of intangible assets									1,563	1,237	192
Interest income, net									(1,799)	(888)	(550)
Income before income taxes	$ 129,038	$ 132,270	$ 130,707	$ 125,501	$ 124,581	$ 146,324	$ 149,414	$ 133,791	517,516	554,110	581,030
Operating segments
Segment Reporting Information [Line Items]
Net service revenues									5,266,789	5,250,399	5,094,933
Operating income									517,280	554,459	580,672
Operating segments | Temporary and consultant staffing
Segment Reporting Information [Line Items]
Net service revenues									4,011,042	4,026,777	3,930,843
Operating income									355,700	393,704	399,808
Operating segments | Permanent placement staffing
Segment Reporting Information [Line Items]
Net service revenues									439,214	419,314	421,411
Operating income									77,673	80,001	85,019
Operating segments | Risk consulting and internal audit services
Segment Reporting Information [Line Items]
Net service revenues									816,533	804,308	742,679
Operating income									83,907	80,754	95,845
Segment reconciling items
Segment Reporting Information [Line Items]
Amortization of intangible assets									1,563	1,237	192
Interest income, net									$ (1,799)	$ (888)	$ (550)