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Summary of Stock Options Outstanding (Detail) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Number of Shares    
Outstanding at January 1, 2019 10,976,118  
Granted 5,613,253  
Exercised (67,461)  
Forfeited (1,395,644)  
Expired (990,585)  
Outstanding at December 31, 2019 14,135,681 10,976,118
Exercisable at December 31, 2019 4,554,661  
Weighted Average Exercise Price per Share    
Outstanding at January 1, 2019 $ 5.75  
Granted 1.32  
Exercised 1.10  
Forfeited 2.25  
Expired 17.07  
Outstanding at December 31, 2019 3.09 $ 5.75
Exercisable at December 31, 2019 $ 6.20  
Weighted Average Remaining Contractual Term (Years)    
Outstanding 7 years 10 months 2 days 7 years 9 months 29 days
Exercisable at December 31, 2019 6 years  
Aggregate Intrinsic Value    
Outstanding at December 31, 2019 $ 182  
Exercisable at December 31, 2019 $ 86