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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, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Expected volatility   67.00%   67.00%
Risk-free interest rate, minimum   1.90%   1.90%
Risk-free interest rate, maximum   2.20%   2.20%
Dividend yield
Minimum [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Expected term in years   6 months   6 months
Maximum [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Expected term in years   2 years   2 years