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Note 5 - Retirement Plans (Details Textual) - USD ($)
$ in Millions
 12 Months Ended 48 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2021
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate 2.80% 2.42%   2.80%
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), Gain (Loss), before Tax $ 851 $ 1,805   $ 851
Defined Benefit Plan, Accumulated Benefit Obligation $ 4,900 $ 5,200   4,900
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% 7.00% 7.00%  
Weighted-average Maturity of Pension Plan Debt Portfolio (Year) 20 years 17 years    
Pension Plan Master Trust, Future Commitments for Additional Contributions, Private Equity Partnerships $ 115 $ 147   115
Pension Plan Master Trust, Future Commitments for Additional Contributions, Real Estate Partnerships and Funds 7 7   $ 7
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Total 85 50 $ 34  
Defined Contribution Plan, Cost 21 19 20  
Railroad Retirement System Contributions 550 569 654  
Collective-Bargaining Arrangement, Other [Member]        
Multiple-Employer Plan Accounted for as Multiemployer Plan, Contribution by Participating Entity $ 30 $ 30 $ 42  
Defined Benefit Plan, Equity Securities [Member] | Minimum [Member]        
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage 45.00%     45.00%
Defined Benefit Plan, Equity Securities [Member] | Maximum [Member]        
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage 55.00%     55.00%
Defined Benefit Plan, Debt Security [Member] | Minimum [Member]        
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage 45.00%     45.00%
Defined Benefit Plan, Debt Security [Member] | Maximum [Member]        
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage 55.00%     55.00%
Pension Plan [Member]        
Actual Return (Loss) on Pension Plan Assets, Net of Fees 15.00% 16.00% 20.00%  
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position, Total $ 258 $ (642)   $ 258
Other Postretirement Benefits Plan [Member]        
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position, Total (165) (190)   $ (165)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Total $ (3) $ (1) $ 10  
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%