Business Segments - Identifiable Assets by Business Segment (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Goodwill	$ 209,793	$ 208,579	$ 199,488
Temporary and consultant staffing
Segment Reporting Information [Line Items]
Goodwill			133,964
Permanent placement staffing
Segment Reporting Information [Line Items]
Goodwill			26,450
Risk consulting and internal audit services
Segment Reporting Information [Line Items]
Goodwill			39,074
Operating segments
Segment Reporting Information [Line Items]
Accounts receivable	736,361	739,727	688,220
Goodwill	209,793	208,579	199,488
Operating segments | Temporary and consultant staffing
Segment Reporting Information [Line Items]
Accounts receivable	403,074	425,179	403,615
Goodwill	133,875	133,173	133,964
Operating segments | Permanent placement staffing
Segment Reporting Information [Line Items]
Accounts receivable	129,506	121,670	115,563
Goodwill	26,015	26,251	26,450
Operating segments | Risk consulting and internal audit services
Segment Reporting Information [Line Items]
Accounts receivable	203,781	192,878	169,042
Goodwill	$ 49,903	$ 49,155	$ 39,074