Annual report pursuant to Section 13 and 15(d)

Stock Plans - Schedule of Fair Value Assumptions (Detail)

v3.8.0.1
Stock Plans - Schedule of Fair Value Assumptions (Detail)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Risk-free interest rate, minimum 1.79% 1.30%
Risk-free interest rate, maximum 2.20% 1.72%
Expected dividend yield 0.00% 0.00%
Employee Stock Purchase Plan [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected term (in years) 6 months  
Expected volatility 53.00%  
Risk-free interest rate 1.15%  
Expected dividend yield 0.00%  
Minimum [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected term (in years) 5 years 9 months 29 days 5 years 6 months 29 days
Expected volatility 52.00% 55.00%
Expected dividend yield 0.00% 0.00%
Maximum [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected term (in years) 9 years 11 months 4 days 6 years 29 days
Expected volatility 65.00% 60.00%
Expected dividend yield 0.00% 0.00%