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Stockholders' Equity (Schedule of Fair Value of Employee Stock Purchase Plan) (Details)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Dividend yield 0.00%    
ESPP [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Expected volatility     70.00%
Risk-free interest rate, minimum 1.30% 0.80%
Risk-free interest rate, maximum   2.70% 1.40%
Dividend yield
ESPP [Member] | Minimum [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Expected term in years   6 months 6 months
Expected volatility   65.00%  
ESPP [Member] | Maximum [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Expected term in years   2 years 2 years
Expected volatility   67.00%