Revenue Recognition - Schedule of Revenue Disaggregated by Line of Business (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) segment	Sep. 30, 2020 USD ($)
Disaggregation of Revenue [Line Items]
Number of reportable segments | segment			3
Guarantee period			90 days
Service revenues	$ 1,712,566	$ 1,189,897	$ 4,691,527	$ 3,804,914
Elimination of Intersegment Revenues
Disaggregation of Revenue [Line Items]
Service revenues	(172,534)	(59,816)	(419,375)	(147,603)
Accountemps
Disaggregation of Revenue [Line Items]
Service revenues	492,558	351,598	1,363,007	1,173,024
OfficeTeam
Disaggregation of Revenue [Line Items]
Service revenues	279,370	173,685	763,035	549,963
Robert Half Technology
Disaggregation of Revenue [Line Items]
Service revenues	215,500	161,007	581,905	519,687
Robert Half Management Resources
Disaggregation of Revenue [Line Items]
Service revenues	239,807	154,917	633,685	531,826
Temporary and consultant staffing
Disaggregation of Revenue [Line Items]
Service revenues	1,054,701	781,391	2,922,257	2,626,897
Permanent placement staffing
Disaggregation of Revenue [Line Items]
Service revenues	156,444	87,203	411,788	278,722
Risk consulting and internal audit services
Disaggregation of Revenue [Line Items]
Service revenues	$ 501,421	$ 321,303	$ 1,357,482	$ 899,295