Business Segments - Identifiable Assets by Business Segment (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Goodwill	$ 208,579	$ 199,488	$ 200,833
Temporary and consultant staffing
Segment Reporting Information [Line Items]
Goodwill	133,173	133,964	134,692
Permanent placement staffing
Segment Reporting Information [Line Items]
Goodwill	26,251	26,450	26,574
Risk consulting and internal audit services
Segment Reporting Information [Line Items]
Goodwill	49,155	39,074	39,567
Operating segments
Segment Reporting Information [Line Items]
Accounts receivable	739,727	688,220	579,166
Goodwill	208,579	199,488	200,833
Operating segments | Temporary and consultant staffing
Segment Reporting Information [Line Items]
Accounts receivable	425,179	403,615	349,364
Goodwill	133,173	133,964	134,692
Operating segments | Permanent placement staffing
Segment Reporting Information [Line Items]
Accounts receivable	121,670	115,563	100,550
Goodwill	26,251	26,450	26,574
Operating segments | Risk consulting and internal audit services
Segment Reporting Information [Line Items]
Accounts receivable	192,878	169,042	129,252
Goodwill	$ 49,155	$ 39,074	$ 39,567