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Stockholders' Equity And Stock-Based Compensation (Schedule Of Fair Value Of Employee Stock Options) (Details)
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Options outstanding [Member]    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Expected volatility 70.00% 70.00%
Risk-free interest rate   2.70%
Risk-free interest rate, minimum 2.40%  
Risk-free interest rate, maximum 2.70%  
Dividend yield
Options outstanding [Member] | Minimum [Member]    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Expected term in years 4 months 24 days 6 years 1 month 6 days
Risk-free interest rate   2.70%
Options outstanding [Member] | Maximum [Member]    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Expected term in years 6 years 1 month 6 days  
2017 Employee Stock Purchase Plan [Member]    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Expected volatility 70.00% 70.00%
Risk-free interest rate, minimum 2.50% 1.90%
Risk-free interest rate, maximum 2.60% 2.10%
Dividend yield
2017 Employee Stock Purchase Plan [Member] | Minimum [Member]    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Expected term in years 6 months 6 months
2017 Employee Stock Purchase Plan [Member] | Maximum [Member]    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Expected term in years 1 year 1 year