Net Income Per Share - Basic and Diluted Net Income Per Share (Detail) - shares	3 Months Ended								12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Earnings Per Share [Abstract]
Weighted average common shares outstanding used in calculating basic income per share									22,269,378	23,966,611	24,203,947
Effect of dilutive options and non-vested restricted stock awards									761,284	712,189	616,900
Weighted average common and common equivalent shares used in calculating diluted income per share	23,286,206	23,182,546	22,940,594	22,712,648	23,591,854	25,095,398	25,116,359	24,923,125	23,030,662	24,678,800	24,820,847