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Stock Compensation Plans - Summary of Restricted Stock Units (Detail) - Restricted stock units - $ / shares
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]    
Unvested, beginning of year (in shares) 7,103  
Granted (in shares) 8,692 8,692
Vested (in shares) (5,402)  
Forfeited (in shares) 0  
Unvested, end of period (in shares) 10,393  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]    
Unvested, beginning of year (in usd per share) $ 17.93  
Granted (in usd per share) 19.56  
Vested (in usd per share) 17.99  
Forfeited (in usd per share) 0.00  
Unvested, end of period (in usd per share) $ 19.26