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Stock Compensation Plans - Summary of Restricted Stock Units (Detail) - Restricted Stock Units
12 Months Ended
Dec. 31, 2017
$ / shares
shares
Shares  
Beginning of period (in shares) | shares 10,259
Granted (in shares) | shares 3,939
Vested (in shares) | shares (8,536)
Forfeited (in shares) | shares 0
End of period (in shares) | shares 5,662
Weighted Average Grant Date Fair Value  
Beginning balance (in usd per share) | $ / shares $ 8.88
Granted (in usd per share) | $ / shares 15.20
Vested (in usd per share) | $ / shares 9.89
Forfeited (in usd per share) | $ / shares 0.00
Ending balance (in usd per share) | $ / shares $ 11.76