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 9 Months Ended 12 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Jun. 30, 2017
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]            
Number of Shares, Options outstanding at beginning of period 376,401   387,901 387,901    
Number of shares, stock options exercised (shares) (10,625) (2,375)   (22,125) (42,584)  
Number of Shares, Options outstanding at end of period 365,776   376,401 365,776   387,901
Number of Shares, Options exercisable at end of period 348,105     348,105    
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) $ 17.68   $ 17.49 $ 17.49    
Weighted-Average Exercise Price Per Share, Options exercised (in usd per share) 12.21     12.19    
Weighted-Average Exercise Price Per Share, Options outstanding at end of period (in usd per share) 17.84   $ 17.68 17.84   $ 17.49
Weighted-Average Exercise Price Per Share, Options exercisable at end of period (in usd per share) $ 17.62     $ 17.62    
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 6 years 1 month   6 years 4 months 6 years 1 month   6 years 10 months
Weighted-Average Remaining Contractual Life, Options exercised 4 years 6 months     4 years 6 months    
Weighted-Average Remaining Contractual Life, Options outstanding at end of period 6 years 1 month   6 years 4 months 6 years 1 month   6 years 10 months
Weighted-Average Remaining Contractual Life, Options exercisable at end of period 6 years 1 month     6 years 1 month    
Aggregate Intrinsic Value of In-the-Money Options, Options outstanding at beginning of period $ 4,168   $ 6,752 $ 6,752    
Aggregate Intrinsic Value of In-the-Money Options, Options outstanding at end of period 4,797   $ 4,168 4,797   $ 6,752
Aggregate Intrinsic Value of In-the-Money Options, Options exercisable at end of period $ 4,641     $ 4,641    
Intrinsic value represents the excess of the closing stock price related to options outstanding (in usd per share) $ 30.95   $ 28.45 $ 30.95   $ 34.90