Cover page - shares	3 Months Ended
	Mar. 31, 2020	Apr. 30, 2020
Cover [Abstract]
Document Type	10-Q/A
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2020
Document Transition Report	false
Entity File Number	1-10427
Entity Registrant Name	ROBERT HALF INTERNATIONAL INC.
Entity Central Index Key	0000315213
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
Amendment Flag	true
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	94-1648752
Entity Address, Address Line One	2884 Sand Hill Road
Entity Address, Address Line Two	Suite 200
Entity Address, City or Town	Menlo Park
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94025
City Area Code	650
Local Phone Number	234-6000
Title of 12(b) Security	Common Stock, par value $0.001 per share
Trading Symbol	RHI
Security Exchange Name	NYSE
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		114,602,242
Amendment Description	Robert Half International Inc. (the “Company”) is filing this Amendment No. 1 on Form 10-Q/A (the “Amendment”) to amend the Company’s quarterly report on Form 10-Q for the period ended March 31, 2020 (the “Form 10-Q”), filed with the Securities and Exchange Commission on May 4, 2020 (the “Original Filing Date”). This Amendment is an exhibit-only filing. The purpose of this Amendment is to correct Exhibit 3.2. which was incorrect on the Form 10-Q. Except for the changes to Exh. 3.2, this Amendment does not otherwise update any exhibits as originally filed or previously amended.No other changes have been made to the Form 10-Q. This Amendment speaks as of the Original Filing Date of the Form 10-Q, does not reflect events that may have occurred subsequent to the Original Filing Date and does not modify or update in any way any other disclosures made in the Form 10-Q. In addition, as required by Rule 12b-15 under the Securities Exchange Act of 1934, as amended (the “Exchange Act”), new certifications by the Company’s principal executive officer and principal financial officer are filed herewith as exhibits to this Amendment pursuant to Rule 13a-14(a) of the Exchange Act. The Company is not including certifications pursuant to Section 1350 of Chapter 63 of Title 18 of the United States Code (18 U.S.C. 1350) as no financial statements are being filed with this Amendment.