Note 2 - Stock Option Valuation Assumptions (Details) - $ / shares |
12 Months Ended | ||
---|---|---|---|
Dec. 31, 2015 |
Dec. 31, 2014 |
Dec. 31, 2013 |
|
Minimum [Member] | |||
Expected volatility | 66.00% | 70.00% | |
Risk-free interest rate | 1.07% | 0.69% | 2.04% |
Expected life (in years) | 3 years | 3 years | 6 years 91 days |
Maximum [Member] | |||
Expected volatility | 71.00% | 73.00% | |
Risk-free interest rate | 2.42% | 2.73% | 3.84% |
Expected life (in years) | 10 years | 10 years | 10 years |
Weighted-average grant date fair value per share (in dollars per share) | $ 17.95 | $ 8.23 | $ 11.37 |
Expected volatility | 73.00% | ||
Dividend yield |