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Income Taxes - Additional Information (Detail) - USD ($)
3 Months Ended 9 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Sep. 30, 2019
Income Tax Disclosure [Abstract]          
Valuation allowance on deferred tax assets $ 164,000   $ 164,000   $ 164,000
Income tax provision $ 11,555,000 $ 6,719,000 $ 16,422,000 $ 10,169,000  
Effective income tax rate 24.90% 26.00% 25.10% 25.80%