Retirement Plan and Other Post-Retirement Benefits - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Defined Benefit Plan Disclosure [Line Items]
Net Periodic Benefit Cost (Income)	$ 9,500	$ 8,300	$ 8,900
Accumulated Benefit Obligation	57,500	58,500	64,900
Projected benefit obligation	$ 66,000	$ 69,500	$ 77,200
Discount rate (as a percent)	4.71%	5.91%	5.49%
Rate of compensation increase (as a percent)	8.00%	8.00%	8.00%
Pre-tax decrease to accumulated other comprehensive income	$ 55,235	$ 788,876	$ (154,986)
Defined Benefit Plan, Tax Status [Extensible Enumeration]	Non-Qualified Benefit Plans	Non-Qualified Benefit Plans	Non-Qualified Benefit Plans
Other Accruals and Current Liabilities
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 14,100	$ 13,100	$ 17,500
Other Liabilities
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	51,900	56,400	59,700
Retirement Savings Account
Defined Benefit Plan Disclosure [Line Items]
Costs recognized	6,500	5,700	5,300
Tax-Deferred Savings Plans
Defined Benefit Plan Disclosure [Line Items]
Costs recognized	9,000	8,200	7,800
Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Net Periodic Benefit Cost (Income)	(5,334)	(4,622)	23,087
Accumulated Benefit Obligation	804,916	751,912	793,555
Projected benefit obligation	$ 822,306	$ 768,750	$ 813,828	$ 1,098,456
Discount rate (as a percent)	4.97%	5.99%	5.57%
Rate of compensation increase (as a percent)	4.60%	4.60%	4.60%
Annual contribution expected for next fiscal year	$ 0
Expected future benefit payments in 2025	67,700
Expected future benefit payments in 2026	67,200
Expected future benefit payments in 2027	66,600
Expected future benefit payments in 2028	65,800
Expected future benefit payments 2029	65,000
Expected future benefit payments 2030 through 2034	305,100
Employer contributions	$ 0	$ 0	$ 20,400
Expected long term rate of return on plan assets (as a percent)	7.40%	6.90%	5.20%
Retirement Plan | Scenario, Plan
Defined Benefit Plan Disclosure [Line Items]
Expected long term rate of return on plan assets (as a percent)	6.60%
Retirement Plan | Other Actuarial Experience
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	$ 2,500
Retirement Plan | Discount Rate Change
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	69,600	$ (28,400)	$ (262,200)
Non-Qualified Benefit Plans, Other Post-Retirement Benefit Plan and Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Increase to other regulatory assets	36,000
Pre-tax decrease to accumulated other comprehensive income	15,000
Other Post-Retirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net Periodic Benefit Cost (Income)	(4,583)	(3,404)	(5,664)
Projected benefit obligation	$ 321,025	$ 274,278	$ 299,283	$ 431,213
Discount rate (as a percent)	4.98%	5.99%	5.56%
Rate of compensation increase (as a percent)	4.60%	4.60%	4.60%
Expected future benefit payments in 2025	$ 25,736
Expected future benefit payments in 2026	26,193
Expected future benefit payments in 2027	26,534
Expected future benefit payments in 2028	26,775
Expected future benefit payments 2029	26,804
Expected future benefit payments 2030 through 2034	128,836
Employer contributions	$ 479	$ 235	$ 3,082
Expected long term rate of return on plan assets (as a percent)	6.00%	5.70%	5.20%
Other Post-Retirement Benefits | Scenario, Plan
Defined Benefit Plan Disclosure [Line Items]
Expected long term rate of return on plan assets (as a percent)	5.40%
Other Post-Retirement Benefits | Other Actuarial Experience
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	$ 1,200	$ (12,900)	$ (22,500)
Other Post-Retirement Benefits | Discount Rate Change
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	28,100	(10,700)	(98,900)
Other Post-Retirement Benefits | Mortality Improvement Projection Scale
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation		(400)	$ 1,100
Other Post-Retirement Benefits | Health Care Cost Trend Rates
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	25,200	$ 3,200
Other Than VEBA Trust And 401(h) Accounts
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	500
VEBA Trust
Defined Benefit Plan Disclosure [Line Items]
Annual contribution expected for next fiscal year	0
Employer contributions	$ 0