| Document And Entity Information | Dec. 10, 2024 |
| Document Information [Line Items] | |
| Entity, Registrant Name | HENNESSY ADVISORS, INC. |
| Document, Type | 8-K |
| Document, Period End Date | Dec. 10, 2024 |
| Entity, Incorporation, State or Country Code | CA |
| Entity, File Number | 001-36423 |
| Entity, Tax Identification Number | 68-0176227 |
| Entity, Address, Address Line One | 7250 Redwood Blvd., Suite 200 |
| Entity, Address, City or Town | Novato |
| Entity, Address, State or Province | CA |
| Entity, Address, Postal Zip Code | 94945 |
| City Area Code | 415 |
| Local Phone Number | 899-1555 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity, Emerging Growth Company | false |
| Amendment Flag | false |
| Entity, Central Index Key | 0001145255 |
| CommonStock Custom [Member] | |
| Document Information [Line Items] | |
| Title of 12(b) Security | Common stock |
| Trading Symbol | HNNA |
| Security Exchange Name | NASDAQ |
| NotesDue20264875 Custom [Member] | |
| Document Information [Line Items] | |
| Title of 12(b) Security | 4.875% Notes due 2026 |
| Trading Symbol | HNNAZ |
| Security Exchange Name | NASDAQ |
