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Note 5 - Retirement Plans (Details Textual) - USD ($) $ in Millions | 12 Months Ended | 60 Months Ended
--- | --- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2022
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate |  | 5.21% | 2.80% |  | 5.21%
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), Gain (Loss), before Tax |  | $ 493 | $ 851 |  | $ 493
Defined Benefit Plan, Accumulated Benefit Obligation |  | $ 3,500 | $ 4,900 |  | 3,500
Defined Benefit Plan, Expected Return on Plan Assets, Changes In Fair Value, Period (Year) |  | 5 years |  |  |
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-Term Rate of Return on Plan Assets |  | 6.25% | 6.25% | 7.00% |
Weighted-average Maturity of Pension Plan Debt Portfolio (Year) |  | 21 years | 20 years |  |
Pension Plan Master Trust, Future Commitments for Additional Contributions, Private Equity Partnerships |  | $ 91 | $ 115 |  | 91
Pension Plan Master Trust, Future Commitments for Additional Contributions, Real Estate Partnerships and Funds |  | 5 | 7 |  | 5
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Total |  | 9 | 85 | $ 50 |
Defined Contribution Plan, Cost |  | 24 | 21 | 19 |
Railroad Retirement System Contributions |  | 586 | 550 | 569 |
Collective-Bargaining Arrangement, Other [Member] |  |  |  |  |
Multiple-Employer Plan Accounted for as Multiemployer Plan, Contribution by Participating Entity |  | $ 20 | $ 30 | $ 30 |
Forecast [Member] |  |  |  |  |
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-Term Rate of Return on Plan Assets | 5.25% |  |  |  |
Forecast [Member] | Defined Benefit Plan, Equity Securities [Member] |  |  |  |  |
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage | 25.00% |  |  |  |
Forecast [Member] | Defined Benefit Plan, Debt Security [Member] |  |  |  |  |
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage | 75.00% |  |  |  |
Pension Plan [Member] |  |  |  |  |
Actual Return (Loss) on Pension Plan Assets, Net of Fees |  | (18.00%) | 15.00% | 16.00% |
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position, Total |  | $ 638 | $ 258 |  | 638
Other Postretirement Benefits Plan [Member] |  |  |  |  |
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position, Total |  | (134) | (165) |  | $ (134)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Total |  | $ (2) | $ (3) | $ (1) |
Four Zero One Thrift Plan Before 2018 [Member] |  |  |  |  |
Defined Contribution Plan, Employer Matching Contribution, Percent of Match |  | 50.00% |  |  |
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay |  | 6.00% |  |  |
Four Zero One Thrift Plan 2018 Beyond [Member] |  |  |  |  |
Defined Contribution Plan, Employer Matching Contribution, Percent of Match |  |  |  |  | 100.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay |  |  |  |  | 6.00%
Defined Contribution Plan, Additional Annual Employer Contribution, Percent of Employees Gross Pay |  | 3.00% |  |  |

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- Definition The expected long-term rate of return on plan assets that is applied to a calculated value of plan assets that recognizes the changes in fair value over this period. + References No definition available. + Details
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- Definition Percentage of employees' gross pay for which the employer contributes in addition to the matching contribution to a defined contribution plan. + References No definition available. + Details
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- Definition The amount of future commitments for additional contributions to private equity partnerships by the Master Trust for a pension plan (defined benefit plan). + References No definition available. + Details
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- Definition The amount of future commitments for additional contributions to real estate partnerships and funds by the Master Trust for a pension plan (defined benefit plan). + References No definition available. + Details
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- Definition Amount of actuarial present value of benefits attributed to employee service rendered, excluding assumptions about future compensation level. + 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 (e) -URI https://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920 + Details
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- Definition Amount, before tax, of accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit. + 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 (j) -URI https://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI https://asc.fasb.org/extlink&oid=126968391&loc=SL7669686-108580 + 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 https://asc.fasb.org/extlink&oid=123447040&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 https://asc.fasb.org/extlink&oid=123447040&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 https://asc.fasb.org/extlink&oid=123447040&loc=d3e2919-114920 + Details
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- Definition Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan. + 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 https://asc.fasb.org/extlink&oid=123450688&loc=d3e4179-114921 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(i) -URI https://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920 + Details
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