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Stockholders' Equity - Common Stock Warrants (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Class of Warrant or Right [Line Items]      
Number of shares underlying outstanding warrants   553,868 553,868
Exercise price per share   $ 0.33 $ 0.33
Number of warrants exercised 546,709 0 23,275
Warrants Exercised Weighted Average Exercise Price   $ 0.33  
Proceeds from cash exercise of warrants $ 2,336    
Number of warrants expired 0   1,544
Common Stock      
Class of Warrant or Right [Line Items]      
Shares issued upon net exercise of outstanding warrants 536,564    
Shares issued upon cash exercise of warrants 7,159