Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Sep. 30, 2016
Income Taxes [Line Items]
Valuation allowance on deferred tax assets	$ 454,000		$ 454,000		$ 454,000
Income tax provision	$ 9,094,000	$ 9,285,000	$ 12,409,000	$ 11,530,000
Effective income tax rate	39.00%	40.20%	38.70%	40.40%
Accounting Standards Update 2016-09 [Member]
Income Taxes [Line Items]
Increase in income tax expense		$ 242,000		$ 376,000