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Document and Entity Information - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Mar. 10, 2025
Jun. 30, 2024
Cover [Abstract]      
Document Type 10-K    
Amendment Flag false    
Document Fiscal Period Focus FY    
Document Period End Date Dec. 31, 2024    
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 3rd 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    
Document Financial Statement Error Correction [Flag] false    
Entity Public Float     $ 181.3
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 2024    
Security Exchange Name NASDAQ    
Entity Common Stock, Shares Outstanding   9,198,175  
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 2025 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    
Auditor Opinion [Text Block]

Opinion on the Consolidated Financial Statements and Financial Highlights

We have audited the accompanying consolidated statements of assets and liabilities of Portman Ridge Finance Corporation and subsidiaries (the "Company"), including the consolidated schedules of investments, as of December 31, 2024 and 2023, the related consolidated statements of operations, cash flows, and changes in net assets for each of the three years in the period then ended, the financial highlights for each of the four years in the period then ended, and the related notes (collectively referred to as “financial statements and financial highlights”). In our opinion, the financial statements and financial highlights present fairly, in all material respects, the financial position of the Company as of December 31, 2024 and 2023, and the results of its operations, changes in net assets, and cash flows for each of the three years in the period then ended, and the financial highlights for each of the four years in the period then ended in conformity with accounting principles generally accepted in the United States of America. The financial highlights for the year ended December 31, 2020 were audited by other auditors whose report dated March 11, 2021, expressed an unqualified opinion on such financial highlights.