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

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- Definition

Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of share based payment arrangements using the treasury stock method.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
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- Definition

The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

+ References

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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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