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Income Taxes - Additional Information (Detail) - USD ($)
3 Months Ended 6 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2019
Mar. 31, 2018
Sep. 30, 2018
Income Taxes [Line Items]            
Valuation allowance on deferred tax assets $ 119,000     $ 119,000   $ 119,000
Income tax provision $ 1,890,000 $ 1,610,000   $ 3,450,000 $ 3,859,000  
Effective income tax rate 26.30% 20.70%   25.30% 27.10%  
Federal corporate tax rate     35.00% 21.00%    
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