Document And Entity Information - USD ($)	12 Months Ended
	Sep. 30, 2025	Dec. 02, 2025	Mar. 31, 2025
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	FY
Document Fiscal Year Focus	2025
Document Type	10-K
Document Annual Report	true
Document Period End Date	Sep. 30, 2025
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
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
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
ICFR Auditor Attestation Flag	false
Document Financial Statement Error Correction [Flag]	false
Entity Shell Company	false
Entity Public Float			$ 49,472,655
Entity Common Stock, Shares Outstanding		7,891,917
Auditor Name	CBIZ CPAs P.C
Auditor Location	San Francisco, CA
Auditor Firm ID	199
Public Income Notes [Member]
Document Information [Line Items]
Title of 12(b) Security	4.875% Notes due 2026
Trading Symbol	HNNAZ
Security Exchange Name	NASDAQ
Common Stock [Member]
Document Information [Line Items]
Title of 12(b) Security	Common stock, no par value
Trading Symbol	HNNA
Security Exchange Name	NASDAQ