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Net Income Per Share - Additional Information (Detail) - shares
3 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Stock Options and Non-Vested Restricted Stock Awards [Member]    
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]    
Anti-dilutive securities excluded from earnings per share calculation 116,844 52,435