Annual report pursuant to Section 13 and 15(d)

Stock Plans - Summary of Fair Value Assumptions of Stock Purchase Plan (Detail)

v3.19.1
Stock Plans - Summary of Fair Value Assumptions of Stock Purchase Plan (Detail) - $ / shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
2018 Performance- Based Stock Incentive Plan [Member] | Chad Steelberg and Ryan Steelberg [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected volatility 73.00%  
Risk-free interest rate 2.85%  
Expected dividend yield 0.00%  
Grant date stock price $ 16.82  
Employee Stock Purchase Plan [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected term (in years) 6 months 6 months
Expected volatility   53.00%
Risk-free interest rate   1.15%
Employee Stock Purchase Plan [Member] | Minimum [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected volatility 50.50%  
Risk-free interest rate 1.18%  
Employee Stock Purchase Plan [Member] | Maximum [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected volatility 53.00%  
Risk-free interest rate 1.89%