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Stockholders' Equity - 2018 Equity Financing - Additional Information (Detail) - USD ($)
 1 Months Ended 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 25, 2018
Jun. 22, 2018
Mar. 28, 2018
Sep. 30, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2018
Dec. 31, 2018
Jun. 30, 2018
Dec. 31, 2019
Jun. 21, 2018
Class of Stock [Line Items]                          
Common Stock, Shares Authorized   150,000,000             150,000,000 150,000,000   150,000,000  
Change in fair value of derivative liability                 $ (18,886,000) $ (45,164,000) $ (26,278,000)    
Exercise price of Second Tranche Warrants $ 1.23           $ 1.23   $ 1.23 $ 1.23 $ 1.23 $ 1.23  
Second Tranche Transaction [Member]                          
Class of Stock [Line Items]                          
Change in fair value of derivative liability           $ (18,900,000) $ (2,300,000) $ (24,000,000)          
First Tranche Securities Purchase Agreement [Member]                          
Class of Stock [Line Items]                          
Common stock issued to investors       8,606,324                  
Common stock price per share       $ 1.10                  
Gross proceeds from issuance of common stock       $ 9,500,000                  
Portion of First Tranche net proceeds allocated to equity       $ 4,600,000                  
Second Securities Purchase Agreement [Member]                          
Class of Stock [Line Items]                          
Gross proceeds to be received from issuance of common shares, subject to stockholder approval     $ 25,500,000                    
2018 Equity Financing [Member]                          
Class of Stock [Line Items]                          
Common Stock, Shares Authorized                         120,000,000
Share issuance costs               343,000          
2018 Equity Financing [Member] | Second Tranche Transaction [Member]                          
Class of Stock [Line Items]                          
Units issued to investors, with each Unit consisting of one common share and one warrant to purchase one common share   20,184,224                      
Unit price per share   $ 1.265                      
Gross proceeds from issuance of units   $ 25,500,000                      
Fair value of derivative liability               6,900,000          
Change in fair value of derivative liability               $ 2,200,000     $ 22,200,000    
Reclassification of the derivative liability balance to equity upon the exercise of the Second Tranche Warrants                     $ 29,100,000    
2018 Equity Financing [Member] | Second Tranche Warrants [Member]                          
Class of Stock [Line Items]                          
Change in fair value of derivative liability $ 1,600,000       $ 18,900,000   $ 18,200,000            
Reclassification of the derivative liability balance to equity upon the exercise of the Second Tranche Warrants         $ 38,700,000                
Exercise price of Second Tranche Warrants         $ 1.43 $ 1.43