Document And Entity Information (USD $)	12 Months Ended
	Sep. 30, 2011	Oct. 25, 2011	Mar. 31, 2011
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Sep. 30, 2011
Document Fiscal Period Focus	FY
Trading Symbol	HNNA
Document Fiscal Year Focus	2011
Entity Registrant Name	HENNESSY ADVISORS INC
Entity Central Index Key	0001145255
Current Fiscal Year End Date	--09-30
Entity Filer Category	Smaller Reporting Company
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Common Stock, Shares Outstanding		5,741,451
Entity Public Float			$ 10,912,367