| Cover | Nov. 16, 2022 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Nov. 15, 2023 |
| Entity File Number | 814-00736 |
| Entity Registrant Name | PennantPark Investment Corporation |
| Entity Central Index Key | 0001383414 |
| Entity Tax Identification Number | 20-8250744 |
| Entity Incorporation, State or Country Code | MD |
| Entity Address, Address Line One | 1691 Michigan Avenue |
| Entity Address, City or Town | Miami Beach |
| Entity Address, State or Province | FL |
| Entity Address, Postal Zip Code | 33319 |
| City Area Code | 786 |
| Local Phone Number | 297-9500 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, par value $0.001 per share |
| Trading Symbol | PNNT |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
