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Note 11 - Pension Plan and Other Postretirement Benefits (Details Textual) - USD ($) | 1 Months Ended | 12 Months Ended
--- | --- | ---
Jan. 31, 2020 | Jul. 30, 2015 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year |  |  | $ 3,300,000 |  |
Defined Benefit Plan, Expected Future Prescription Drug Subsidy Receipt, Next Twelve Months |  |  | 100,000 |  |
Defined Contribution Plan, Cost |  |  | 5,265,000 | $ 4,532,000 | $ 4,211,000
Employee Stock Ownership Plan (ESOP), Cash Contributions to ESOP |  |  | $ 374,000 | 398,000 | $ 612,000
Pension Plan [Member] |  |  |  |  |
Defined Benefit Plan, Percentage Vesting After the Specified Employment Period |  |  | 100.00% |  |
Defined Benefit Plan, Vesting Period |  |  | 5 years |  |
Defined Benefit Plan, Age for Compensation |  |  | 65 |  |
Defined Benefit Plan, Age Below Which Compensation is Reduced |  |  | 62 |  |
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets for Next Fiscal Year |  |  | 6.88% |  |
Defined Benefit Plan, Plan Assets at Fair Value, Valuation Inputs, Period Over Which Investment Gains or Losses are Recognized |  |  | 5 years |  |
Defined Benefit Plan, Plan Assets, Contributions by Employer |  |  | $ 22,500,000 | 20,000,000 |
Pension Plan [Member] | SEI Energy Debt Fund [Member] |  |  |  |  |
Amount Invested in Pension Fund Assets |  | $ 10,000,000.0 |  |  |
Pension Plan [Member] | Subsequent Event [Member] |  |  |  |  |
Defined Benefit Plan, Plan Assets, Contributions by Employer | $ 11,200,000 |  |  |  |
Other Postretirement Benefits Plan [Member] |  |  |  |  |
Defined Benefit Plan, Plan Assets, Contributions by Employer |  |  | $ 2,757,000 | 3,183,000 |
Health Insurance Benefits, Requisite Age |  |  | 55 years |  |
Health Insurance Benefits, Requisite Service Period |  |  | 10 years |  |
Health Insurance Benefits, Reduction in Prescription Reimbursement, Percent |  |  | 27.00% |  |
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment |  |  | $ (20,864,000) | $ 0 |
Health Insurance Benefits, Reduction in Benefits, Period for Recognition |  |  | 4 years 3 months 18 days |  |

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- Definition Represents the age below which compensation is reduced under a defined benefit plan. + References No definition available. + Details
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- Definition Represents the expected long-term return on pension fund assets used for the determination of the next fiscal year's net periodic benefit cost under a defined benefit plan. + References No definition available. + Details
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- Definition Represents the percentage vesting under a defined benefit plan after the specified employment period (vesting period). + References No definition available. + Details
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- Definition Represents the duration of the period over which investment gains or losses are recognized, from the year in which they occur, in the calculation of the fair value of a defined benefit plan's assets. + References No definition available. + Details
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- Definition Represents the vesting period of a defined benefit plan. + References No definition available. + Details
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- Definition Amount of contribution received by defined benefit plan from employer which increases plan assets. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 6 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2709-114920 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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