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Stockholders' Equity (Schedule of Fair Value of Employee Stock Options) (Details) - Stock Options [Member]
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Expected term 5 years 2 months 12 days 6 years 1 month 6 days 5 years 2 months 12 days 6 years 1 month 6 days
Expected volatility 65.00% 70.00% 67.00% 70.00%
Risk-free interest rate 2.80% 1.90% 2.50% 2.10%
Dividend yield