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Stockholders' Equity (Schedule Of Fair Value Of Employee Stock Options) (Details) - Stock Options [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 term 6 years 1 month 6 days 6 years 1 month 6 days 6 years 1 month 6 days
Expected volatility 70.00% 70.00% 70.00%
Risk-free interest rate 2.10% 1.50% 1.70%
Dividend yield