Business Segments - Reconciliation of Revenue and Operating Income by Reportable Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Net service revenues	$ 1,395,333	$ 1,287,370
Operating income	134,367	125,579
Amortization of intangible assets	463	301
Interest income, net	(735)	(223)
Income before income taxes	134,639	125,501
Operating Segments | Temporary and consultant staffing
Segment Reporting Information [Line Items]
Net service revenues	1,066,288	987,606
Operating income	96,723	90,371
Operating Segments | Permanent placement staffing
Segment Reporting Information [Line Items]
Net service revenues	121,400	103,633
Operating income	22,379	18,302
Operating Segments | Risk consulting and internal audit services
Segment Reporting Information [Line Items]
Net service revenues	207,645	196,131
Operating income	$ 15,265	$ 16,906