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Stockholders' Equity (Schedule Of Fair Value Of Employee Stock Purchase Plan) (Details) - ESPP [Member]
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
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 1.10% 0.50% 0.80% 0.50%
Risk-free interest rate, maximum 1.40% 0.80% 1.40% 0.90%
Dividend yield
Minimum [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Expected term 6 months 6 months 6 months 6 months
Maximum [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Expected term 2 years 2 years 2 years 2 years