Business Segments - Reconciliation of Revenue and Operating Income by Reportable Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Net service revenues	$ 1,324,709	$ 1,338,541	$ 3,920,507	$ 3,985,326
Operating income	132,221	146,367	388,517	429,762
Amortization of intangible assets	481	326	1,147	928
Interest income, net	(530)	(283)	(1,108)	(695)
Income before income taxes	132,270	146,324	388,478	429,529
Operating Segments | Temporary and consultant staffing
Segment Reporting Information [Line Items]
Net service revenues	1,004,808	1,016,248	2,990,710	3,057,496
Operating income	87,703	97,786	271,162	302,802
Operating Segments | Permanent placement staffing
Segment Reporting Information [Line Items]
Net service revenues	111,333	105,117	327,619	324,845
Operating income	19,945	20,286	59,293	66,364
Operating Segments | Risk consulting and internal audit services
Segment Reporting Information [Line Items]
Net service revenues	208,568	217,176	602,178	602,985
Operating income	$ 24,573	$ 28,295	$ 58,062	$ 60,596