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Stock Compensation Plans - Summary of Restricted Stock Units (Detail) - Restricted stock units - $ / shares
3 Months Ended 9 Months Ended
Mar. 31, 2018
Sep. 30, 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) 5,662 5,662
Granted (in shares) 8,989 8,989
Vested (in shares)   (7,339)
Forfeited (in shares)   (24)
Unvested, end of period (in shares)   7,288
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) $ 11.76 $ 11.76
Granted (in usd per share)   18.50
Vested (in usd per share)   13.90
Forfeited (in usd per share)   15.20
Unvested, end of period (in usd per share)   $ 17.91