Note 4 - Financial Instruments - Instruments by Significant Investment Category (Details) - Fair Value, Inputs, Level 1 [Member] - USD ($) $ in Thousands |
12 Months Ended | |
|---|---|---|
Dec. 31, 2021 |
Dec. 31, 2020 |
|
| Adjusted cost | $ 31,659 | $ 41,012 |
| Unrealized Gains | 0 | 15 |
| Unrealized Losses | (6) | 0 |
| Market Value | 31,653 | 41,027 |
| Money Market Funds [Member] | ||
| Adjusted cost | 1,670 | 6,035 |
| Unrealized Gains | 0 | 0 |
| Unrealized Losses | 0 | 0 |
| Market Value | 1,670 | 6,035 |
| US Treasury Securities Considered Cash Equivalents [Member] | ||
| Adjusted cost | 0 | 9,998 |
| Unrealized Gains | 0 | 0 |
| Unrealized Losses | 0 | 0 |
| Market Value | 0 | 9,998 |
| US Treasury Securities in Short-term Investments [Member] | ||
| Adjusted cost | 29,989 | 24,979 |
| Unrealized Gains | 0 | 15 |
| Unrealized Losses | (6) | 0 |
| Market Value | $ 29,983 | $ 24,994 |