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Stockholders' Equity And Stock-Based Compensation (Schedule Of Fair Value Of Employee Stock Options) (Details)
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Minimum [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Expected term in years 5 years 3 months 18 days 5 years 3 months 18 days 4 months 24 days 5 years 3 months 18 days
Risk-free interest rate 1.90% 2.70% 1.90% 2.70%
Maximum [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Expected term in years 6 years 1 month 6 days 6 years 1 month 6 days 6 years 1 month 6 days 6 years 1 month 6 days
Risk-free interest rate 2.40% 3.00% 2.60% 3.00%
Options outstanding [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Expected volatility 70.00% 70.00% 70.00% 70.00%
Dividend yield
2017 Employee Stock Purchase Plan [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Expected volatility 70.00% 70.00% 70.00% 70.00%
Risk-free interest rate   1.90%   1.90%
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 6 months 6 months
Risk-free interest rate 2.50%   2.50%  
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 1 year 1 year
Risk-free interest rate 2.60%   2.60%