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 ($)
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Expected term (years) 5 years 8 months 12 days 4 years 6 months
Expected volatility 78.30% 74.00%
Risk-free interest rate 2.50% 2.50%
Expected dividend yield 0.00% 0.00%
Weighted-average estimated grant date fair value per share $ 25.66 $ 20.39
Options granted 928,675 719,950
Total estimated grant date fair value $ 23,830,000 $ 14,680,000