Quarterly report pursuant to Section 13 or 15(d)

Stock Based Compensation - Weighted-Average Fair Value Valuation Assumptions (Detail)

v3.19.2
Stock Based Compensation - Weighted-Average Fair Value Valuation Assumptions (Detail) - Employee Stock Options - $ / shares
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Risk-free interest rate 2.31% 2.81% 2.55% 2.79%
Expected term (in years) 6 years 29 days 6 years 10 days 6 years 29 days 6 years 10 days
Dividend yield 0.00% 0.00% 0.00% 0.00%
Volatility 108.89% 107.00% 108.87% 107.00%
Weighted-average fair value of stock options granted $ 6.65 $ 5.20 $ 4.28 $ 5.83