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Net Income Per Share - Additional Information (Detail) - shares
3 Months Ended 6 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Mar. 31, 2018
Mar. 31, 2017
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 0 7,417 0 15,918