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Note 6 - Other Assets (Details Textual) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Cash Surrender Value, Fair Value Disclosure | $ 13,700 | $ 11,600 |
Increase (Decrease) in Cash Surrender Value | 2,400 | (400) |
Investments Held in Rabbi Trust [Member] |  |  |
Debt Securities, Available-for-sale, Amortized Cost, Total | 1,100 | 1,400 |
Debt Securities, Available-for-sale, Total | 1,100 | 1,400 |
Supplemental Employee Retirement Plan [Member] |  |  |
Defined Benefit Plan, Benefit Obligation, Ending Balance | 21,541 | 18,676 | $ 19,134
Investments, Fair Value Disclosure, Total | $ 14,700 | $ 13,000 |

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