Cover Page - shares	3 Months Ended
	Dec. 31, 2021	Feb. 07, 2022
Document Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Dec. 31, 2021
Document Fiscal Year Focus	2022
Document Fiscal Period Focus	Q1
Entity Registrant Name	HENNESSY ADVISORS, INC.
Entity Central Index Key	0001145255
Current Fiscal Year End Date	--09-30
Entity Current Reporting Status	Yes
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 Common Stock, Shares Outstanding		7,478,103
Document Transition Report	false
Document Quarterly Report	true
Entity Address, Address Line One	7250 Redwood Boulevard, Suite 200
Entity Address, City or Town	Novato
Entity File Number	001-36423
Entity Tax Identification Number	68-0176227
City Area Code	415
Local Phone Number	899-1555
Entity Address, Postal Zip Code	94945
Entity Incorporation, State or Country Code	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