| Cover | Jun. 25, 2024 |
| Document Information [Line Items] | |
| Document Type | 8-K |
| Document Period End Date | Jun. 25, 2024 |
| Entity Registrant Name | Portman Ridge Finance Corporation |
| Entity Incorporation, State or Country Code | DE |
| Securities Act File Number | 814-00735 |
| Entity Tax Identification Number | 20-5951150 |
| 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 |
| City Area Code | 212 |
| Local Phone Number | 891-2880 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001372807 |
| Amendment Flag | false |
| Title of 12(b) Security | Common Stock, par value $0.01 per share |
| Trading Symbol | PTMN |
| Security Exchange Name | NASDAQ |
| N-2 - shares | Jun. 25, 2024 | Apr. 26, 2024 |
| Cover [Abstract] | ||
| Entity Central Index Key | 0001372807 | |
| Amendment Flag | false | |
| Securities Act File Number | 814-00735 | |
| Document Type | 8-K | |
| Entity Registrant Name | Portman Ridge Finance Corporation | |
| 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 | |
| City Area Code | 212 | |
| Local Phone Number | 891-2880 | |
| Entity Emerging Growth Company | false | |
| Common Shares [Member] | ||
| Capital Stock, Long-Term Debt, and Other Securities [Abstract] | ||
| Outstanding Security, Title [Text Block] | common stock | |
| Outstanding Security, Not Held [Shares] | 9,311,908 |
