Revenue Recognition - Schedule of Revenue Disaggregated by Line of Business (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($) segment	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Disaggregation of Revenue [Line Items]
Number of reportable segments | segment									3
Guarantee period									90 days
Service revenues	$ 1,304,086	$ 1,189,897	$ 1,108,326	$ 1,506,691	$ 1,537,385	$ 1,552,132	$ 1,516,385	$ 1,468,530	$ 5,109,000	$ 6,074,432	$ 5,800,271
Elimination of intersegment revenues
Disaggregation of Revenue [Line Items]
Service revenues									(239,996)	(172,439)	(132,381)
Accountemps
Disaggregation of Revenue [Line Items]
Service revenues									1,558,024	1,985,221	1,941,544
OfficeTeam
Disaggregation of Revenue [Line Items]
Service revenues									764,947	1,040,755	1,065,427
Robert Half Technology
Disaggregation of Revenue [Line Items]
Service revenues									695,418	765,831	717,166
Robert Half Management Resources
Disaggregation of Revenue [Line Items]
Service revenues									698,942	792,757	738,810
Temporary and consultant staffing
Disaggregation of Revenue [Line Items]
Service revenues									3,477,335	4,412,125	4,330,566
Permanent placement staffing
Disaggregation of Revenue [Line Items]
Service revenues									370,109	533,432	511,989
Risk consulting and internal audit services
Disaggregation of Revenue [Line Items]
Service revenues									$ 1,261,556	$ 1,128,875	$ 957,716