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Stockholders' Equity (Schedule of Fair Value of Employee Stock Options) (Details) - Stock Options [Member]
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Expected term (years) 5 years   5 years 2 months 12 days
Expected volatility 70.70%   66.80%
Risk-free interest rate 0.30%   2.60%
Dividend yield