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Stockholders' Equity - Weighted-Average assumptions (Detail) - 2013 Equity Incentive Plan - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Weighted-average fair value of the options granted $ 4.76 $ 3.39 $ 5.08
Weighted-average assumptions      
Expected term 6 years 3 months 6 years 3 months 6 years 3 months
Expected volatility 75.20% 68.98% 80.76%
Risk-free interest rate 2.08% 1.70% 1.70%
Expected dividend yield 0.00% 0.00% 0.00%