v3.26.1
Document And Entity Information - shares
9 Months Ended
Jun. 30, 2026
Aug. 03, 2026
Document Information [Line Items]    
Entity Central Index Key 0001145255  
Entity Registrant Name HENNESSY ADVISORS INC  
Amendment Flag false  
Current Fiscal Year End Date --09-30  
Document Fiscal Period Focus Q3  
Document Fiscal Year Focus 2026  
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Jun. 30, 2026  
Document Transition Report false  
Entity File Number 001-36423  
Entity Incorporation, State or Country Code CA  
Entity Tax Identification Number 68-0176227  
Entity Address, Address Line One 7250 Redwood Boulevard, 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  
Title of 12(b) Security Common stock, no par value  
Trading Symbol HNNA  
Security Exchange Name NASDAQ  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Non-accelerated Filer  
Entity Small Business true  
Entity Emerging Growth Company false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   7,904,410