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Stock-Based Compensation (Assumptions Used in Estimation of Fair Value of Stock) (Details) - $ / shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Weighted-average Black-Scholes fair value of stock options granted $ 1.74 $ 1.06 $ 1.88
Risk-free interest rate, minimum 2.26% 1.61% 0.97%
Risk-free interest rate, maximum 3.10% 2.34% 1.78%
Dividend yield 0.00% 0.00% 0.00%
Volatility, minimum 93.31% 88.91% 57.86%
Volatility, maximum 115.61% 114.10% 108.88%
Employee Stock Purchase Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Risk-free interest rate, minimum 0.66% 0.45% 0.22%
Risk-free interest rate, maximum 2.24% 1.13% 0.61%
Dividend yield 0.00% 0.00% 0.00%
Volatility, minimum 52.19% 45.98% 43.03%
Volatility, maximum 203.83% 267.85% 86.75%
Expected forfeiture rate [1]     5.00%
Minimum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected term (in years) 4 years 25 days 4 years 1 month 20 days 4 years 2 months 19 days
Expected forfeiture rate [1]     0.00%
Minimum [Member] | Employee Stock Purchase Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Weighted-average Black-Scholes fair value of stock options granted $ 0.36 $ 0.45 $ 1.86
Expected term (in years) 6 months 6 months 6 months
Maximum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected term (in years) 7 years 6 months 7 years 5 months 16 days 7 years 3 months 11 days
Expected forfeiture rate [1]     16.33%
Maximum [Member] | Employee Stock Purchase Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Weighted-average Black-Scholes fair value of stock options granted $ 3.53 $ 5.47 $ 4.76
Expected term (in years) 2 years 2 years 2 years
[1] See Note 3 regarding the Company’s adoption of ASU 2016-09 in 2017.