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Warrants - Outstanding Warrants To Purchase Common Stock Warrants (Detail) - Common Stock [Member]
6 Months Ended
Jun. 30, 2019
$ / shares
shares
Class Of Warrant Or Right [Line Items]  
Number of Shares Underlying Warrants 466,695
Class Of Warrant Or Right Issued on August 2012 [Member]  
Class Of Warrant Or Right [Line Items]  
Issue Date 2012-08
Classified as Equity
Exercise Price | $ / shares $ 21.78
Number of Shares Underlying Warrants 111,455
Class Of Warrant Or Right Issued on August 2012 [Member] | Remaining Term (in Years) [Member]  
Class Of Warrant Or Right [Line Items]  
Original Term 7 years
Class Of Warrant Or Right Issued on January 2015 [Member]  
Class Of Warrant Or Right [Line Items]  
Issue Date 2015-01
Classified as Equity
Exercise Price | $ / shares $ 6.96
Number of Shares Underlying Warrants 34,483
Class Of Warrant Or Right Issued on January 2015 [Member] | Remaining Term (in Years) [Member]  
Class Of Warrant Or Right [Line Items]  
Original Term 5 years
Class Of Warrant Or Right Issued on April 2016 [Member]  
Class Of Warrant Or Right [Line Items]  
Issue Date 2016-04
Classified as Liability
Exercise Price | $ / shares $ 1.12
Number of Shares Underlying Warrants 320,757
Class Of Warrant Or Right Issued on April 2016 [Member] | Remaining Term (in Years) [Member]  
Class Of Warrant Or Right [Line Items]  
Original Term 7 years