Business Segments - Reconciliation of Revenue and Operating Income by Reportable Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Net service revenues	$ 1,287,370	$ 1,302,625
Operating income	125,579	133,898
Amortization of intangible assets	301	288
Interest income, net	(223)	(181)
Income before income taxes	125,501	133,791
Operating Segments | Temporary and consultant staffing
Segment Reporting Information [Line Items]
Net service revenues	987,606	1,009,165
Operating income	90,371	97,883
Operating Segments | Permanent placement staffing
Segment Reporting Information [Line Items]
Net service revenues	103,633	106,289
Operating income	18,302	21,502
Operating Segments | Risk consulting and internal audit services
Segment Reporting Information [Line Items]
Net service revenues	196,131	187,171
Operating income	$ 16,906	$ 14,513