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Stockholders' Equity (Schedule Of Fair Value Of Employee Stock Purchase Plan) (Details)
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Risk-free interest rate, maximum     1.30%  
ESPP [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, minimum 0.80% 0.50% 0.80% 0.50%
Risk-free interest rate, maximum 1.30% 0.90%   0.90%
Dividend yield
ESPP [Member] | Minimum [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Expected term 6 months 6 months 6 months 6 months
ESPP [Member] | Maximum [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Expected term 2 years 2 years 2 years 2 years