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Company and Summary of Significant Accounting Policies - Stock-Based Compensation (Details) - Equity Incentive 2011 Plan [Member]
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Significant Accounting Policies [Line Items]        
Expected term (in years) 6 years 6 years 6 years 6 years
Weighted average volatility (percentage) 120.67% 116.64% 120.14% 108.56%
Weighted average risk free interest rate (percentage) 0.93% 0.42% 0.94% 0.46%
Expected dividends 0.00% 0.00% 0.00% 0.00%
Weighted-average expected forfeiture rate (percentage) 11.03% 8.79% 12.25% 8.64%