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Stockholders' Equity (Schedule of Fair Value of Employee Stock Purchase Plan) (Details)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
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% 70.00%
Risk-free interest rate, minimum 1.30% 0.80% 0.50%
Risk-free interest rate, maximum 2.70% 1.40% 0.90%
Dividend yield
ESPP [Member] | Minimum [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Expected term (years) 6 months 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 (years) 2 years 2 years 2 years
Expected volatility 67.00%