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Document and Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2022
Mar. 08, 2023
Jun. 30, 2022
Cover [Abstract]      
Document Type 10-K    
Amendment Flag false    
Document Fiscal Period Focus FY    
Document Period End Date Dec. 31, 2022    
Entity Registrant Name Portman Ridge Finance Corporation    
Current Fiscal Year End Date --12-31    
Entity Central Index Key 0001372807    
Entity File Number 814-00735    
Entity Tax Identification Number 20-5951150    
Entity Current Reporting Status Yes    
Entity Voluntary Filers No    
Entity Address, Address Line One 650 Madison Avenue    
Entity Address, Address Line Two 23rd Floor    
Entity Address, City or Town New York    
Entity Address, State or Province NY    
Entity Address, Postal Zip Code 10022    
Entity Incorporation, State or Country Code DE    
Entity Filer Category Non-accelerated Filer    
Entity Well-known Seasoned Issuer No    
ICFR Auditor Attestation Flag false    
Entity Public Float     $ 221,736,014
Entity Emerging Growth Company false    
Entity Shell Company false    
Entity Small Business false    
Trading Symbol PTMN    
City Area Code 212    
Local Phone Number 891-2880    
Title of 12(b) Security Common Stock, par value $0.01 per share    
Entity Interactive Data Current Yes    
Document Fiscal Year Focus 2022    
Security Exchange Name NASDAQ    
Entity Common Stock, Shares Outstanding   9,551,548  
Document Transition Report false    
Document Annual Report true    
Documents Incorporated by Reference [Text Block]

DOCUMENTS INCORPORATED BY REFERENCE

Portions of the registrant’s definitive proxy statement for the 2023 annual meeting of stockholders to be filed with the Securities and Exchange Commission within 120 days following the end of the registrant’s fiscal year-end are incorporated by reference into Part III of this report.

    
Auditor Name Deloitte & Touche LLP    
Auditor Firm ID 34    
Auditor Location New York