Stock-based Compensation - Schedule of Estimated Fair Value of Option Award Granted Determined Using Black-Scholes Option-Pricing Valuation Model (Detail) |
3 Months Ended | 6 Months Ended | ||
---|---|---|---|---|
Jun. 30, 2018 |
Jun. 30, 2017 |
Jun. 30, 2018 |
Jun. 30, 2017 |
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Disclosure of Compensation Related Costs, Share-based Payments [Abstract] | ||||
Expected volatility | 86.80% | 90.60% | 85.90% | 90.90% |
Risk-free interest rate | 2.80% | 1.90% | 2.80% | 2.20% |
Expected dividend yield | 0.00% | 0.00% | 0.00% | 0.00% |
Expected term (years) | 5 years 9 months 18 days | 6 years | 5 years 4 months 24 days | 5 years 8 months 12 days |