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Stockholders' Equity - Weighted-Average assumptions (Detail) - $ / shares
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Weighted-average assumptions    
Expected term 6 years 3 months 6 years 3 months
2013 Equity Incentive Plan    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Weighted-average fair value of the options granted $ 13.70 $ 4.17
Weighted-average assumptions    
Expected volatility 73.89% 74.95%
Risk-free interest rate 2.42% 2.11%
Expected dividend yield 0.00% 0.00%