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Equity Plans - Employee Stock Purchase Plan (Details) - shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Jul. 31, 2016
ESPP rights        
Equity Plans        
Expected term 6 months 6 months 6 months  
Expected volatility, Minimum 89.10% 58.90% 49.00%  
Expected volatility, Maximum 120.40% 65.30% 63.40%  
Risk-free interest rate, Minimum 0.12% 1.89% 1.89%  
Risk-free interest rate, Maximum 0.43% 2.32% 2.32%  
2016 ESPP        
Equity Plans        
Maximum payroll deduction for share purchases (as a percent) 15.00%      
Purchase price of stock (as a percent) 85.00%      
Shares issued in period   92,523    
Number of shares available for issuance   757,647    
Number of shares authorized       150,000
Maximum annual additional authorized shares as a percentage of outstanding shares 1.00%      
Maximum annual additional authorized shares (in shares) 300,000