Note 8 - Stockholders' Equity - Valuation Assumptions (Details) |
12 Months Ended | ||
---|---|---|---|
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
|
Employee Stock Option [Member] | |||
Risk-free interest rate | 2.10% | 1.70% | 1.70% |
Dividend yield | |||
Volatility | 74.00% | 89.10% | 92.70% |
Expected life (years) (Year) | 6 years | 6 years | 6 years |
Employee Stock Purchase Plan [Member] | |||
Risk-free interest rate | 1.20% | 0.40% | 0.10% |
Dividend yield | |||
Volatility | 50.00% | 68.30% | 86.70% |
Expected life (years) (Year) | 180 days | 180 days | 180 days |