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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%