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Summary of Non-Vested Shares (Detail) - $ / shares
3 Months Ended 6 Months Ended
Jun. 30, 2018
Mar. 31, 2018
Jun. 30, 2017
Mar. 31, 2017
Jun. 30, 2018
Jun. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Number of Shares, Vested         (19,313) (24,312)
Common Stock Grants            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Number of Shares, Beginning balance 632,660 391,753 559,587 324,756 391,753 324,756
Number of Shares, Granted 22,308 254,972 38,502 251,475    
Number of Shares, Vested (20,313) (8,800) (188,400) (10,100)    
Number of Shares, Forfeited (6,424) (5,265) (6,593) (6,544)    
Number of Shares, Ending Balance 628,231 632,660 403,096 559,587 628,231 403,096
Weighted Average Grant-Date Fair Value, Beginning balance $ 17.41 $ 15.61 $ 15.38 $ 14.75 $ 15.61 $ 14.75
Weighted Average Grant-Date Fair Value, Granted 23.66 19.97 17.08 16.10    
Weighted Average Grant-Date Fair Value, Vested 22.82 12.07 15.22 12.95    
Weighted Average Grant-Date Fair Value, Forfeited 17.25 16.51 15.55 15.26    
Weighted Average Grant-Date Fair Value, Ending balance $ 17.40 $ 17.41 $ 15.61 $ 15.38 $ 17.40 $ 15.61