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Net Income Per Share - Additional Information (Detail) - shares
12 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2018
Stock Options [Member]      
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]      
Anti-dilutive securities excluded from earnings per share calculation 9,650 10,988 1,288