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Share-Based Compensation - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Option to purchase voting common stock (shares)     596,995  
Cash received from exercise of stock options     $ 0.3  
Tax benefit from exercise of stock options     0.2  
Share-based compensation expense $ 1.2 $ 1.1 $ 3.6 $ 3.1
Common Stock | Director        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Deferred compensation shares issued     21,993  
Noninterest expense directors fees     $ 0.6  
Stock Option        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Option to purchase voting common stock (shares)     596,995  
Vesting period (years)     5 years  
Expiration period (years)     10 years  
Service period (years)     5 years  
Minimum total shareholder return (percent)     50.00%  
Minimum average annual growth rate in EPS (percent)     10.00%  
Restricted Stock Units        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Vesting period (years)     3 years  
Restricted stock units granted     158,581  
Restricted Stock Units | Bonus Recognition and Retention Program        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Vesting period (years)     5 years  
Restricted stock units granted     84,392