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Stockholders' Equity - Summary of Restricted Stock Activity (Detail) - Restricted Stock - $ / shares
shares in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Stock Awards, Shares      
Non vested balance, beginning of period (in shares) 436    
Granted (in shares) 323    
Vested (in shares) (207)    
Forfeited (in shares) (9)    
Non vested balance, end of period (in shares) 543 436  
Stock Awards, Weighted-Average Grant Date Fair Value      
Non vested balance, beginning of period (USD per share) $ 17.83    
Granted (USD per share) 15.25 $ 17.32 $ 18.24
Vested (USD per share) 18.77    
Forfeitured (USD per share) 16.44    
Non vested balance, end of period (USD per share) $ 16.23 $ 17.83