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Stockholders' Equity (Schedule Of Fair Value Of Employee Stock Purchase Plan) (Details) - ESPP [Member]
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Expected volatility 70.00% 70.00% 70.00%
Risk-free interest rate, minimum 0.80% 0.50% 0.10%
Risk-free interest rate, maximum 1.40% 0.90% 0.70%
Dividend yield
Minimum [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Expected term 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