Summary of the Organization, Description of Business and Significant Accounting Policies - Additional Information (Detail) (USD $)	0 Months Ended	1 Months Ended	12 Months Ended			0 Months Ended	1 Months Ended	12 Months Ended		12 Months Ended
	Jan. 17, 2013	Oct. 26, 2012 Funds	Sep. 30, 2013 Funds	Sep. 30, 2012 Funds	Sep. 30, 2011	Nov. 05, 2013 FBR Funds [Member]	Oct. 26, 2012 FBR Funds [Member]	Sep. 30, 2013 FBR Funds [Member]	Oct. 28, 2013 FBR Funds [Member]	Sep. 30, 2013 Stock Appreciation Right [Member]	Sep. 30, 2013 Incentive Stock Option Plan [Member]	Sep. 30, 2013 Restricted Stock Units (RSUs) [Member]	Sep. 30, 2013 Minimum [Member]	Sep. 30, 2013 Maximum [Member]
Partnership Organization And Basis Of Presentation [Line Items]
Number of Hennessy Funds			16
Unrealized gains			$ 3,000	$ 7,000
Number of mutual funds				13
Management contracts impairment amount			0	0
Number of funds acquired		10
Acquisition of FBR & Co. (FBRC) deal		2,200,000,000
Additional loan borrowed		16,300,000						13,286,666
Payments to acquire management contract rights through available cash		3,400,000
Additional capitalized transaction cost			1,200,000	200,000
Initial payment based upon the net asset value of the FBR Funds							19,692,137
Contingent payment based upon the net asset value of the FBR Funds			19,193,595			19,193,595
Contingent payment due under the Asset Purchase Agreement									19,193,595
Long term liability			11,625,883					11,625,883
Estimated useful lives of assets													P1Y	P10Y
Unrecognized tax benefits			0	0
Interest and penalties related to unrecognized tax benefits			0	0
Effective tax rates			40.90%	43.90%
Principal reasons for differences from the federal statutory rate			34.00%
Common stock equivalents (antidilutive) excluded from per share calculation			245,998	433,438
Shares available for issuance	50.00%
Share based compensation percentage criteria description	More than 50% of the outstanding shares
Aggregate number of shares										50,000
Date of grant remaining term of option														10 years
Stock option granted to fair value											10.00%
Percentage of exercise price of fair value													110.00%
Maximum Vesting Period												4 years		5 years
Maximum aggregate fair market value of shares			100,000
Shares grant period			10 years
Option Reserve for Common Stock			2,043,791
Option vested and exercisable			185,998
Weighted average exercise price			$ 7.09
Number of Options Exercised			247,440	5,000
Number of options fully vested and exercisable			185,998	433,438	645,050
Restricted stock units, granted												60,000
Rate of restricted stock units vest under plan												25.00%
compensation expense related to non-vested awards not yet recognized			$ 500,000
Recognition of Compensation Expenses			4 years