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Retirement Plans, Weighted-Average Assumption Used to Determine Benefit Obligation (Details) | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019
Pension Plan [Member] |  |  |  |
Weighted-average assumption used to determine benefit obligation at end of year [Abstract] |  |  |  |
Discount rate |  | 2.80% | [1] | 3.50%
Health care cost trend rate [Abstract] |  |  |  |
Initial rate |  | 0.00% |  | 0.00%
Ultimate rate |  | 0.00% |  | 0.00%
Pension Plan [Member] | Higman [Member] |  |  |  |
Weighted-average assumption used to determine benefit obligation at end of year [Abstract] |  |  |  |
Discount rate | [1] | 2.90% |  |
SERP [Member] |  |  |  |
Weighted-average assumption used to determine benefit obligation at end of year [Abstract] |  |  |  |
Discount rate |  | 2.80% |  | 3.50%
Rate of compensation increase |  | 0.00% |  | 0.00%
Health care cost trend rate [Abstract] |  |  |  |
Initial rate |  | 0.00% |  | 0.00%
Ultimate rate |  | 0.00% |  | 0.00%
Other Postretirement Benefits [Member] |  |  |  |
Weighted-average assumption used to determine benefit obligation at end of year [Abstract] |  |  |  |
Discount rate |  | 2.80% |  | 3.50%
Rate of compensation increase |  | 0.00% |  | 0.00%
Health care cost trend rate [Abstract] |  |  |  |
Initial rate |  | 6.50% |  | 6.75%
Ultimate rate |  | 5.00% |  | 5.00%
Years to ultimate |  | 2025 |  | 2025
[1] | The 2020 discount rate was 2.8% for the Kirby pension plan and 2.9% for the Higman pension plan.

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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 Ultimate trend rate for health care cost for defined benefit postretirement 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 (l) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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- Definition Year ultimate health care cost trend rate is expected to be reached, in CCYY format. + 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 (l) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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