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Stockholders' Equity (Schedule of Fair Value of Employee Stock Options) (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%    
Stock Options [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Expected term (years) 5 years 2 months 12 days 6 years 1 month 6 days 6 years 1 month 6 days
Expected volatility 66.80% 70.00% 70.00%
Risk-free interest rate 2.60% 2.10% 1.50%
Dividend yield