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Note 2 - Marketable Securities (Tables) | 12 Months Ended
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Dec. 31, 2018
Notes Tables |
Schedule of Available-for-sale Securities Reconciliation [Table Text Block] |
|  | Amortized cost |  |  | Gross unrealized holding gains |  |  | Gross unrealized holding losses |  |  | Estimated fair value |
December 31, 201 8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Available-for-sale securities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate bonds and notes |  | $ | 2,911 |  |  | $ | 1 |  |  | $ | (31 | ) |  | $ | 2,881 |
Municipal bonds |  |  | 1,849 |  |  |  | — |  |  |  | (15 | ) |  |  | 1,834 |
Total available-for-sale securities |  | $ | 4,760 |  |  | $ | 1 |  |  | $ | (46 | ) |  | $ | 4,715 |
December 31, 201 7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Available-for-sale securities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate bonds and notes |  | $ | 8,458 |  |  | $ | 19 |  |  | $ | (49 | ) |  | $ | 8,428 |
Municipal bonds |  |  | 4,637 |  |  |  | 1 |  |  |  | (28 | ) |  |  | 4,610 |
Total available-for-sale securities |  | $ | 13,095 |  |  | $ | 20 |  |  | $ | (77 | ) |  | $ | 13,038 |
Available-for-sale Securities, Debt Maturities [Table Text Block] |
|  | Amortized |  |  | Estimated |
|  | cost |  |  | fair value |
Due within one year |  | $ | 955 |  |  | $ | 951 |
Due after one year through five years |  |  | 3,805 |  |  |  | 3,764 |
Total available-for-sale securities |  | $ | 4,760 |  |  | $ | 4,715 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Table Text Block] |
|  | Less than 12 months |  |  | More than 12 months |  |  | Total |
|  | Estimated fair value |  |  | Gross unrealized holding losses |  |  | Estimated fair value |  |  | Gross unrealized holding losses |  |  | Estimated fair value |  |  | Gross unrealized holding losses |
As of December 31, 201 8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate bonds and notes |  | $ | — |  |  | $ | — |  |  | $ | 2,881 |  |  | $ | 31 |  |  | $ | 2,881 |  |  | $ | 31 |
Municipal bonds |  |  | — |  |  |  | — |  |  |  | 1,834 |  |  |  | 15 |  |  |  | 1,834 |  |  |  | 15 |
|  | $ | — |  |  | $ | — |  |  | $ | 4,715 |  |  | $ | 46 |  |  | $ | 4,715 |  |  | $ | 46 |

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