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Stock Plans and Stock-Based Compensation - Assumptions used to Calculate Fair Value of Stock-Based Compensation (Details)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]      
Expected term (in years) 10 years    
Dividend yield 0.00%    
Employee Stock Option      
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]      
Risk-free interest rate, minimum 0.40% 1.60% 1.20%
Risk-free interest rate, maximum 0.50% 2.30% 3.10%
Expected term (in years) 6 years 7 days 6 years 7 days  
Volatility, minimum 76.50% 71.80% 58.70%
Volatility, maximum 78.50% 74.60% 75.50%
Dividend yield 0.00% 0.00% 0.00%
ESPP      
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]      
Risk-free interest rate, minimum 0.10% 1.60% 2.10%
Risk-free interest rate, maximum 0.20% 2.30% 2.60%
Volatility, minimum 66.60% 64.80% 54.30%
Volatility, maximum 136.00% 77.10% 65.50%
Dividend yield 0.00% 0.00% 0.00%
Minimum | Employee Stock Option      
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]      
Expected term (in years)     5 years 1 month 28 days
Minimum | ESPP      
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]      
Expected term (in years) 6 months 6 months 6 months
Maximum | Employee Stock Option      
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]      
Expected term (in years)     9 years 11 months 12 days
Maximum | ESPP      
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]      
Expected term (in years) 2 years 2 years 2 years