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