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Stockholders' Equity (Schedule of Fair Value of Employee Stock Purchase Plan) (Details) - ESPP [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 volatility    
Risk-free interest rate    
Risk-free interest rate, minimum 0.10%   1.30%
Risk-free interest rate, maximum 1.00%   2.70%
Dividend yield
Minimum [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Expected term (years) 6 months   6 months
Expected volatility 57.00%   65.00%
Maximum [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Expected term (years) 2 years   2 years
Expected volatility 71.00%   67.00%