Cover Page - USD ($)	12 Months Ended
	Dec. 31, 2019	Jan. 31, 2020	Jun. 30, 2019
Cover page.
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2019
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	2019
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	94-1648752
Entity Address, Address Line One	2884 Sand Hill Road
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 $.001 per Share
Trading Symbol	RHI
Security Exchange Name	NYSE
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
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 Public Float			$ 6,518,872,985
Entity Common Stock, Shares Outstanding		115,120,403
Documents Incorporated by Reference	Portions of the registrant’s Proxy Statement to be mailed to stockholders in connection with the registrant’s annual meeting of stockholders, scheduled to be held in May 2020, are incorporated by reference in Part III of this report. Except as expressly incorporated by reference, the registrant’s Proxy Statement shall not be deemed to be part of this report.