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Note 8 - Benefit Plans - Assumptions Used to Determine Benefit Obligations and Net Periodic Benefit (Details) | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Pension Plan [Member] | Corporate Segment [Member] |  |  |
Benefit obligations, discount rate | 2.36% | 3.12% |
Benefit obligations, long term rate of return | 8.00% | 8.00% |
Benefit cost, discount rate | 3.12% | 4.14% | 3.53%
Benefit cost, long term rate of return | 8.00% | 8.00% | 8.00%
Pension Plan [Member] | Plants [Member] |  |  |
Benefit obligations, discount rate | 2.21% | 3.02% |
Benefit obligations, long term rate of return | 8.00% | 8.00% |
Benefit cost, discount rate | 3.02% | 4.06% | 3.45%
Benefit cost, long term rate of return | 8.00% | 8.00% | 8.00%
Supplemental Employee Retirement Plan [Member] |  |  |
Benefit obligations, discount rate | 2.36% | 3.12% |
Benefit obligations, salary scale | 3.00% | 3.00% |
Benefit cost, discount rate | 3.12% | 4.14% | 3.53%
Benefit cost, salary scale | 3.00% | 3.00% | 3.00%

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- Definition Defined benefit plan assumptions used calculating benefit obligations expected long term return on assets + References No definition available. + Details
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- Definition Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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- Definition Weighted average rate increase of compensation, used to determine benefit obligation of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(2) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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- Definition Weighted average rate for present value of future retirement benefits cash flows, used to determine net periodic benefit cost of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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- Definition Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(3) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2919-114920 + Details
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- Definition Weighted average rate of compensation increase used to determine net periodic benefit cost of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(2) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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