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Summary of Non-Vested Shares (Detail) - Common Stock Grants - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of Shares, Beginning balance 324,756 362,841  
Number of Shares, Granted 290,977 150,009  
Number of Shares, Vested (203,165) (152,479)  
Number of Shares, Forfeited (20,815) (35,615)  
Number of Shares, Ending Balance 391,753 324,756 362,841
Weighted Average Grant-Date Fair Value, Beginning balance $ 14.75 $ 20.43  
Weighted Average Grant-Date Fair Value, Granted 16.24 15.22 $ 12.68
Weighted Average Grant-Date Fair Value, Vested 15.14 15.19  
Weighted Average Grant-Date Fair Value, Forfeited 15.29 18.89  
Weighted Average Grant-Date Fair Value, Ending balance $ 15.63 $ 14.75 $ 20.43