Goodwill (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Goodwill [Roll Forward]
Beginning balance	$ 208,579	$ 199,488
Acquisitions	1,547	10,988
Foreign currency translation adjustments	(333)	(1,897)
Ending balance	209,793	208,579
Temporary and consultant staffing
Goodwill [Roll Forward]
Beginning balance		133,964
Acquisitions	1,248	0
Foreign currency translation adjustments	(546)	(791)
Permanent placement staffing
Goodwill [Roll Forward]
Beginning balance		26,450
Acquisitions	0	0
Foreign currency translation adjustments	(236)	(199)
Risk consulting and internal audit services
Goodwill [Roll Forward]
Beginning balance		39,074
Acquisitions	299	10,988
Foreign currency translation adjustments	449	(907)
Operating segments
Goodwill [Roll Forward]
Beginning balance	208,579	199,488
Ending balance	209,793	208,579
Operating segments | Temporary and consultant staffing
Goodwill [Roll Forward]
Beginning balance	133,173	133,964
Ending balance	133,875	133,173
Operating segments | Permanent placement staffing
Goodwill [Roll Forward]
Beginning balance	26,251	26,450
Ending balance	26,015	26,251
Operating segments | Risk consulting and internal audit services
Goodwill [Roll Forward]
Beginning balance	49,155	39,074
Ending balance	$ 49,903	$ 49,155