Quarterly report pursuant to Section 13 or 15(d)

Share-Based Compensation - Summary of Stock Options Outstanding under Plans (Detail)

v3.8.0.1
Share-Based Compensation - Summary of Stock Options Outstanding under Plans (Detail) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 6 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Jun. 30, 2017
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]        
Number of Shares, Options outstanding at beginning of period 364,088      
Number of shares, stock options exercised (shares) (25,750) (1,000)    
Number of Shares, Options outstanding at end of period 338,338      
Number of Shares, Options exercisable at end of period 329,003      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]        
Weighted-Average Exercise Price Per Share, Options outstanding at beginning of period (in usd per share) $ 17.86      
Weighted-Average Exercise Price Per Share, Options exercised (in usd per share) 22.06      
Weighted-Average Exercise Price Per Share, Options outstanding at end of period (in usd per share) 17.54      
Weighted-Average Exercise Price Per Share, Options exercisable at end of period (in usd per share) $ 17.37      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term [Abstract]        
Weighted-Average Remaining Contractual Life, Options outstanding at beginning of period 5 years 6 months   5 years 10 months 24 days  
Weighted-Average Remaining Contractual Life, Options exercised 5 years 10 months 24 days      
Weighted-Average Remaining Contractual Life, Options outstanding at end of period 5 years 6 months   5 years 10 months 24 days  
Weighted-Average Remaining Contractual Life, Options exercisable at end of period 5 years 4 months 24 days      
Aggregate Intrinsic Value of In-the-Money Options, Options outstanding at beginning of period $ 4,548      
Aggregate Intrinsic Value of In-the-Money Options, Options outstanding at end of period 4,469      
Aggregate Intrinsic Value of In-the-Money Options, Options exercisable at end of period $ 4,625      
Intrinsic value represents the excess of the closing stock price related to options outstanding (in usd per share) $ 30.75     $ 30.35