Goodwill (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2020 USD ($)
Goodwill [Roll Forward]
Balance as of December 31, 2019	$ 210,364
Foreign currency translation adjustments	(534)
Balance as of June 30, 2020	209,830
Temporary and consulting staffing
Goodwill [Roll Forward]
Balance as of December 31, 2019	134,210
Foreign currency translation adjustments	(245)
Balance as of June 30, 2020	133,965
Permanent placement staffing
Goodwill [Roll Forward]
Balance as of December 31, 2019	26,097
Foreign currency translation adjustments	(66)
Balance as of June 30, 2020	26,031
Risk consulting and internal audit services
Goodwill [Roll Forward]
Balance as of December 31, 2019	50,057
Foreign currency translation adjustments	(223)
Balance as of June 30, 2020	$ 49,834