Business Segments - Reconciliation of Revenue and Operating Income by Reportable Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Net service revenues	$ 1,466,226	$ 1,324,709	$ 4,318,613	$ 3,920,507
Operating income	151,138	132,221	435,016	388,517
Amortization of intangible assets	429	481	1,334	1,147
Interest income, net	(1,196)	(530)	(2,937)	(1,108)
Income before income taxes	151,905	132,270	436,619	388,478
Operating Segments | Temporary and consultant staffing
Segment Reporting Information [Line Items]
Net service revenues	1,083,766	1,004,808	3,238,028	2,990,710
Operating income	102,707	87,703	303,000	271,162
Operating Segments | Permanent placement staffing
Segment Reporting Information [Line Items]
Net service revenues	129,667	111,333	386,105	327,619
Operating income	22,895	19,945	72,710	59,293
Operating Segments | Risk consulting and internal audit services
Segment Reporting Information [Line Items]
Net service revenues	252,793	208,568	694,480	602,178
Operating income	$ 25,536	$ 24,573	$ 59,306	$ 58,062