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Company and Summary of Significant Accounting Policies - Stock-Based Compensation (Details) - Equity Incentive 2011 Plan [Member]
3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Significant Accounting Policies [Line Items]        
Expected term (in years) 6 years 6 years 6 years 6 years
Weighted average volatility (percentage) 120.35% 101.38% 119.43% 101.40%
Weighted average risk free interest rate (percentage) 1.12% 0.44% 0.95% 0.49%
Expected dividends 0.00% 0.00% 0.00% 0.00%
Weighted-average expected forfeiture rate (percentage) 10.23% 8.21% 13.89% 8.50%