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Note 11 - Pension Plan and Other Postretirement Benefits - Pension Fund Assets Measured at Fair Value and NAV (Details) - Pension Plan [Member] - USD ($) $ in Thousands | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | --- | ---
Fair Value of Plan Assets | $ 329,781 | $ 269,783 | $ 285,319
NAV Practical Expedient to Fair Valuation [Member] | Other - SEI Energy Debt Collective Fund [Member] | SEI Energy Debt Collective Fund [Member] |  |  |
Fair Value of Plan Assets | 9,540 | 11,476 |
Fair Value, Inputs, Level 1 [Member] |  |  |
Fair Value of Plan Assets | $ 320,241 | $ 258,307 |

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