Quarterly report pursuant to Section 13 or 15(d)

Stockholders' Equity - Summary of Estimated Weighted-Average Assumptions (Detail)

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Stockholders' Equity - Summary of Estimated Weighted-Average Assumptions (Detail) - Employees - USD ($)
9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Expected term (years) 5 years 10 months 24 days 4 years 7 months 6 days
Expected volatility 76.60% 73.40%
Risk-free interest rate 2.20% 2.70%
Expected dividend yield 0.00% 0.00%
Weighted-average estimated grant date fair value per share $ 20.03 $ 22.86
Options granted 2,100,225 2,633,950
Total estimated grant date fair value $ 42,068,000 $ 60,212,000