Business Segments - Reconciliation of Revenue and Operating Income by Reportable Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment Reporting Information [Line Items]
Net service revenues	$ 1,338,541	$ 1,312,718	$ 3,985,326	$ 3,790,339
Operating income	146,367	159,066	429,762	436,315
Amortization of intangible assets	326	0	928	0
Interest income, net	(283)	(240)	(695)	(400)
Income before income taxes	146,324	159,306	429,529	436,715
Operating Segments | Temporary and consultant staffing
Segment Reporting Information [Line Items]
Net service revenues	1,016,248	999,822	3,057,496	2,922,942
Operating income	97,786	102,991	302,802	300,404
Operating Segments | Permanent placement staffing
Segment Reporting Information [Line Items]
Net service revenues	105,117	110,748	324,845	319,744
Operating income	20,286	24,377	66,364	67,460
Operating Segments | Risk consulting and internal audit services
Segment Reporting Information [Line Items]
Net service revenues	217,176	202,148	602,985	547,653
Operating income	$ 28,295	$ 31,698	$ 60,596	$ 68,451