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Retirement and Postretirement Employee Benefit Plans - Actuarial Assumptions For Net Periodic Pension Cost (Details) - Postretirement Plan
12 Months Ended
Sep. 30, 2024
Sep. 30, 2023
Sep. 30, 2022
Defined Benefit Plan Disclosure [Line Items]      
Discount rate pension liability 5.01% 6.06%  
Discount rate postretirement cost 6.06% 5.61% 3.01%
Expected return on plan assets pension liability 5.14% 4.94%  
Expected return on plan assets postretirement cost 4.94% 4.94% 4.94%
Initial trend rate pension liability 6.75% 6.50%  
Initial trend rate postretirement cost 6.50% 6.25% 6.25%
Ultimate trend rate pension liability 5.00% 5.00%  
Ultimate trend rate postretirement cost 5.00% 4.75% 5.00%