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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
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