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Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended			6 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Sep. 30, 2017
Income Taxes [Line Items]
Valuation allowance on deferred tax assets	$ 299,000			$ 299,000		$ 246,000
Income tax provision	$ 1,610,000		$ 1,484,000	$ 3,859,000	$ 3,315,000
Effective income tax rate	20.70%		35.10%	27.10%	38.10%
Federal corporate tax rate	21.00%	35.00%
Tax Cuts and Jobs Act [Member]
Income Taxes [Line Items]
Reduction in beginning deferred tax assets				$ 889,000
Increase in income tax provision				$ 889,000

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Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

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