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Summary of Non-Vested Shares (Detail) - Common Stock Grants - $ / shares
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Number of Shares, Beginning balance 391,753 324,756
Number of Shares, Granted 254,972 251,475
Number of Shares, Vested (8,800) (10,100)
Number of Shares, Forfeited (5,265) (6,544)
Number of Shares, Ending Balance 632,660 559,587
Weighted Average Grant-Date Fair Value, Beginning balance $ 15.63 $ 14.75
Weighted Average Grant-Date Fair Value, Granted 19.97 16.10
Weighted Average Grant-Date Fair Value, Vested 12.07 12.95
Weighted Average Grant-Date Fair Value, Forfeited 16.51 15.26
Weighted Average Grant-Date Fair Value, Ending balance $ 17.42 $ 15.38