Annual report pursuant to Section 13 and 15(d)

Fair Value of Warrants - Assumptions Used in Computing Fair Value of Derivative Warrants (Detail)

v3.20.1
Fair Value of Warrants - Assumptions Used in Computing Fair Value of Derivative Warrants (Detail)
Dec. 31, 2019
$ / shares
Jan. 09, 2019
$ / shares
Dec. 31, 2018
$ / shares
Class of Warrant or Right [Line Items]      
Exercise price (usd per share) $ 104.18 $ 7.43  
2017 Debt      
Class of Warrant or Right [Line Items]      
Exercise price (usd per share) $ 84.60    
Expected life (years) 5 years 11 months 20 days    
2016 Offerings      
Class of Warrant or Right [Line Items]      
Exercise price (usd per share) $ 67.50   $ 67.50
Expected life (years) 2 years 29 days   3 years 29 days
2017 Offerings      
Class of Warrant or Right [Line Items]      
Exercise price (usd per share)     $ 70.80
Expected life (years)     5 months 7 days
Expected volatility | 2017 Debt      
Class of Warrant or Right [Line Items]      
Measurement input assumption 0.7340    
Expected volatility | 2016 Offerings      
Class of Warrant or Right [Line Items]      
Measurement input assumption 1.5069   1.0051
Expected volatility | 2017 Offerings      
Class of Warrant or Right [Line Items]      
Measurement input assumption     1.7250
Risk-free interest rate | 2017 Debt      
Class of Warrant or Right [Line Items]      
Measurement input assumption 0.0255    
Risk-free interest rate | 2016 Offerings      
Class of Warrant or Right [Line Items]      
Measurement input assumption 0.0158   0.0246
Risk-free interest rate | 2017 Offerings      
Class of Warrant or Right [Line Items]      
Measurement input assumption     0.0256
Expected dividend yield | 2017 Debt      
Class of Warrant or Right [Line Items]      
Measurement input assumption 0    
Expected dividend yield | 2016 Offerings      
Class of Warrant or Right [Line Items]      
Measurement input assumption 0   0
Expected dividend yield | 2017 Offerings      
Class of Warrant or Right [Line Items]      
Measurement input assumption     0