Goodwill (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021 USD ($)
Goodwill [Roll Forward]
Balance as of December 31, 2020	$ 223,055
Foreign currency translation adjustments	(9)
Balance as of March 31, 2021	223,046
Temporary and consultant staffing
Goodwill [Roll Forward]
Balance as of December 31, 2020	134,511
Foreign currency translation adjustments	(46)
Balance as of March 31, 2021	134,465
Permanent placement staffing
Goodwill [Roll Forward]
Balance as of December 31, 2020	26,180
Foreign currency translation adjustments	(14)
Balance as of March 31, 2021	26,166
Risk consulting and internal audit services
Goodwill [Roll Forward]
Balance as of December 31, 2020	62,364
Foreign currency translation adjustments	51
Balance as of March 31, 2021	$ 62,415