| Document and Entity Information | Aug. 09, 2023 |
| Cover [Abstract] | |
| Entity Registrant Name | Portman Ridge Finance Corp |
| Amendment Flag | false |
| Entity Central Index Key | 0001372807 |
| Document Type | 8-K |
| Document Period End Date | Aug. 09, 2023 |
| Entity Incorporation State Country Code | DE |
| Entity 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 |
| Security 12b Title | Common Stock, par value $0.01 per share |
| Trading Symbol | PTMN |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
