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Stockholders' Equity - Equity Offerings (Detail) - USD ($)
$ / shares in Units, $ in Millions
 1 Months Ended 9 Months Ended
Oct. 01, 2017
May 31, 2017
Sep. 30, 2017
At The Market Equity Offering | Subsequent Event      
Equity offerings      
Issuance of common stock, net of issuance costs, shares 1,600,000    
Weighted average price per share issued $ 12.50    
Aggregate net proceeds from stock sale $ 19.2    
At The Market Equity Offering | Common Stock      
Equity offerings      
Issuance of common stock, net of issuance costs, shares     1,388,647
Weighted average price per share issued     $ 5.550
Aggregate net proceeds from stock sale     $ 7.4
Public Equity Offering | Common Stock      
Equity offerings      
Issuance of common stock, net of issuance costs, shares   8,050,000  
Weighted average price per share issued   $ 11.50  
Aggregate net proceeds from stock sale   $ 86.8