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Net Income Per Share - Additional Information (Detail) - shares
12 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2015
Stock Options [Member]      
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]      
Anti-dilutive securities excluded from earnings per share calculation 18,526 140,521 1,553,207