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Fair value measurements - Schedule Of weighted-average to determine the fair value of warrants (Detail) - Black Scholes Model [Member] - $ / shares
1 Months Ended 9 Months Ended
Jan. 31, 2019
Oct. 31, 2019
Redeemable Series A Preferred Stock [Member] | Fair Value Warrants [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Estimated fair value of preferred stock $ 5.80  
Exercise price $ 2.88  
Remaining term 7 years 3 days  
Risk-free interest rate 2.60%  
Expected volatility 45.10%  
Dividend yield 0.00%  
Redeemable Series A Preferred Stock [Member] | Fair Value Warrant Liability Member [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Estimated fair value of preferred stock   $ 18.00
Exercise price   $ 6.33
Remaining term   6 years 6 months 18 days
Risk-free interest rate   1.90%
Expected volatility   45.90%
Dividend yield   0.00%
Junior Preferred Stock [Member] | Fair Value Warrants [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Estimated fair value of preferred stock $ 4.88  
Exercise price $ 0.01  
Remaining term 1 year 7 months 6 days  
Risk-free interest rate 2.50%  
Expected volatility 45.10%  
Dividend yield 0.00%  
Junior Preferred Stock [Member] | Fair Value Warrant Liability Member [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Estimated fair value of preferred stock   $ 18.00
Exercise price   $ 0.01
Remaining term   1 year 1 month 17 days
Risk-free interest rate   1.90%
Expected volatility   45.90%
Dividend yield   0.00%
Redeemable Preferred Stock [Member] | Fair Value Warrants [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Estimated fair value of preferred stock $ 0.01  
Exercise price $ 0.01  
Remaining term 1 year 7 months 6 days  
Risk-free interest rate 2.50%  
Expected volatility 45.10%  
Dividend yield 0.00%