| Cover | Oct. 20, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Oct. 20, 2021 |
| Entity File Number | 001-36423 |
| Entity Registrant Name | HENNESSY ADVISORS, INC. |
| Entity Central Index Key | 0001145255 |
| Entity Tax Identification Number | 68-0176227 |
| Entity Incorporation, State or Country Code | CA |
| 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 |
| Title of 12(b) Security | Common stock, no par value |
| Trading Symbol | HNNA |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
