Retirement and Postretirement Employee Benefit Plans - Actuarial Assumptions For Net Periodic Pension Cost (Details) - Postretirement Plan	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Defined Benefit Plan Disclosure [Line Items]
Discount rate pension liability	6.06%	5.61%
Discount rate postretirement cost	5.61%	3.01%	2.80%
Expected return on plan assets pension liability	4.94%	4.94%
Expected return on plan assets postretirement cost	4.94%	4.94%	4.94%
Initial trend rate pension liability	6.50%	6.25%
Initial trend rate postretirement cost	6.25%	6.25%	6.25%
Ultimate trend rate pension liability	5.00%	4.75%
Ultimate trend rate postretirement cost	4.75%	5.00%	5.00%