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Stock-based Payment Arrangements - Unvested Restricted Stock Activity (Detail) - Restricted Stock Awards - $ / shares
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Number of Shares      
Number of Shares, Beginning Balance (shares) 55,380 54,635 48,557
Granted, Number of Shares (shares) 18,745 16,345 18,660
Forfeited, Number of Shares (shares) (4,251) (195) (400)
Released, Number of Shares (shares) (18,711) (15,405) (12,182)
Number of Shares, Ending Balance (shares) 51,163 55,380 54,635
Weighted-Average Grant Date Fair Value      
Weighted-Average Grant Date Fair Value, Beginning Balance (in usd per share) $ 34.36 $ 29.26 $ 24.89
Granted, Weighted-Average Grant Date Fair Value (in usd per share) 35.84 44.88 37.02
Forfeited, Weighted-Average Grant Date Fair Value (in usd per share) 36.01 30.79 28.84
Released, Weighted-Average Grant Date Fair Value (in usd per share) 31.79 27.46 23.76
Weighted-Average Grant Date Fair Value, Ending Balance (in usd per share) $ 35.71 $ 34.36 $ 29.26