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Stockholders' Equity (Schedule of Fair Value of Employee Stock Purchase Plan) (Details) - ESPP [Member]
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Expected volatility 67.00% 70.00% 67.00% 70.00%
Risk-free interest rate, minimum 1.90% 0.80% 1.90% 0.80%
Risk-free interest rate, maximum 2.20% 1.30% 2.20% 1.30%
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