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Note 6 - Other Assets (Details Textual) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Cash Surrender Value, Fair Value Disclosure | $ 14,700 | $ 13,700 |
Increase (Decrease) in Cash Surrender Value | 1,400 | 2,400 |
Investments Held in Rabbi Trust [Member] |  |  |
Debt Securities, Available-for-sale, Amortized Cost, Total | 700 | 1,100 |
Debt Securities, Available-for-sale, Total | 700 | 1,100 |
Supplemental Employee Retirement Plan [Member] |  |  |
Defined Benefit Plan, Benefit Obligation, Ending Balance | 24,308 | 21,541 | $ 18,676
Investments, Fair Value Disclosure, Total | $ 15,400 | $ 14,700 |

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