| Document and Entity Information | Aug. 29, 2022 |
| Document And Entity Information [Line Items] | |
| Entity Registrant Name | HENNESSY ADVISORS INC |
| Amendment Flag | false |
| Entity Central Index Key | 0001145255 |
| Document Type | 8-K |
| Document Period End Date | Aug. 29, 2022 |
| Entity Incorporation State Country Code | CA |
| Entity File Number | 001-36423 |
| Entity Tax Identification Number | 68-0176227 |
| Entity Address, Address Line One | 7250 Redwood Blvd. |
| Entity Address, Address Line Two | 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 |
| Common Stock [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | Common stock, no par value |
| Trading Symbol | HNNA |
| Security Exchange Name | NASDAQ |
| Four Point Eight Seven Five Percent Notes Due Two Thousand Twenty Six [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 4.875% Notes due 2026 |
| Trading Symbol | HNNAZ |
| Security Exchange Name | NASDAQ |
