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Stockholders' Equity - Schedule of Weighted-Average Estimated Value of Employee Stock Options Granted (Detail)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Equity [Abstract]    
Expected volatility 85.00% 91.00%
Risk-free interest rate 2.42% 1.81%
Expected dividends 0.00% 0.00%
Expected term in years 5 years 10 months 24 days 4 years 9 months 18 days