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Stockholders' Equity - Additional Information (Detail) - USD ($)
 12 Months Ended
Jan. 08, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
May 05, 2016
Stockholders Equity Note [Line Items]          
Proceeds from common stock sold   $ 30,197,000 $ 22,121,000 $ 75,300,000  
Preferred stock, shares issued   0 0    
Stockholders rights plan expiration date   Oct. 27, 2019      
Stockholder Rights Plan          
Stockholders Equity Note [Line Items]          
Minimum percentage of common stock acquired by stockholders   15.00%      
Designated preferred stock for future issuance   250,000      
Series C Junior Participating preferred stock          
Stockholders Equity Note [Line Items]          
Preferred stock, shares issued   0      
Cantor | Sales Agreement | Amendment No. 2          
Stockholders Equity Note [Line Items]          
Maximum common stock offering price   $ 132,200,000      
Common stock registered for sale         $ 70,000,000
Common stock, number of shares issued   11,000,000 3,500,000    
Proceeds from common stock sold   $ 30,300,000 $ 22,000,000    
Cantor | Sales Agreement | Amendment No. 3 | Subsequent Event          
Stockholders Equity Note [Line Items]          
Maximum common stock offering price $ 70,000,000        
Percentage of proceeds payable as compensation to underwriter 2.00%        
Unsold shares of common stock, value $ 31,400,000