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Equity Plans and Stock-Based Compensation - Fair Value-Based Measurements and Weighted-Average Assumptions (Detail) - $ / shares
3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Stock Options        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Weighted-average fair value per share $ 3.77 $ 4.92 $ 3.61 $ 4.67
Risk-free interest rate 0.30% 2.30% 1.20% 2.20%
Expected life (in years) 4 years 6 months 4 years 6 months 4 years 6 months 4 years 6 months
Volatility 0.90% 0.90% 0.90% 0.90%
Employee Stock Purchase Plan        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Weighted-average fair value per share     $ 2.17 $ 5.19
Risk-free interest rate     1.50% 2.50%
Expected life (in years)     1 year 2 months 12 days 1 year 2 months 12 days
Volatility     0.60% 0.80%