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Summary of Non-Vested Shares (Detail) - Common Stock Grants - $ / shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of Shares, Beginning balance 391,753 324,756  
Number of Shares, Granted 282,030 290,977  
Number of Shares, Vested (53,304) (203,165)  
Number of Shares, Forfeited (33,269) (20,815)  
Number of Shares, Ending Balance 587,210 391,753 324,756
Weighted Average Grant-Date Fair Value, Beginning balance $ 15.61 $ 14.75  
Weighted Average Grant-Date Fair Value, Granted 20.21 16.24 $ 15.22
Weighted Average Grant-Date Fair Value, Vested 17.06 15.14  
Weighted Average Grant-Date Fair Value, Forfeited 17.29 15.29  
Weighted Average Grant-Date Fair Value, Ending balance $ 17.59 $ 15.61 $ 14.75