v3.22.2.2
Cover Page - USD ($)
 12 Months Ended
Sep. 30, 2022
Dec. 05, 2022
Mar. 31, 2022
Document Information [Line Items]      
Document Type 10-K    
Amendment Flag false    
Document Period End Date Sep. 30, 2022    
Document Fiscal Year Focus 2022    
Document Fiscal Period Focus FY    
Entity Registrant Name HENNESSY ADVISORS, INC.    
Entity Central Index Key 0001145255    
Current Fiscal Year End Date --09-30    
Entity Well-known Seasoned Issuer No    
Entity Current Reporting Status Yes    
Entity Voluntary Filers No    
Entity Filer Category Non-accelerated Filer    
Entity Emerging Growth Company false    
Entity Small Business true    
Entity Shell Company false    
Entity Address, State or Province CA    
Entity Interactive Data Current Yes    
Entity Public Float     $ 45,182,641
Entity Common Stock, Shares Outstanding   7,573,706  
Document Transition Report false    
Document Annual Report true    
ICFR Auditor Attestation Flag false    
Entity File Number 001-36423    
Entity Address, Address Line One 7250 Redwood Boulevard, Suite 200    
Entity Address, City or Town Novato    
Entity Address, Postal Zip Code 94945    
Entity Incorporation, State or Country Code CA    
Entity Tax Identification Number 68-0176227    
City Area Code 415    
Local Phone Number 899-1555    
Auditor Name Marcum LLP    
Auditor Firm ID 688    
Auditor Location Costa Mesa, CA    
Common Class A [Member]      
Document Information [Line Items]      
Trading Symbol HNNA    
Title of 12(b) Security Common stock, no par value    
Security Exchange Name NASDAQ    
4.875% Notes due 2026 [Member]      
Document Information [Line Items]      
Trading Symbol HNNAZ    
Title of 12(b) Security 4.875% Notes due 2026    
Security Exchange Name NASDAQ