Retirement Plan and Other Post-Retirement Benefits - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Defined Benefit Plan Disclosure [Line Items]
Net Periodic Benefit Cost (Income)	$ 8,300	$ 8,900	$ 8,300
Accumulated Benefit Obligation	58,500	64,900	76,900
Projected benefit obligation	$ 69,500	$ 77,200	$ 95,800
Discount rate (as a percent)	5.91%	5.49%	2.15%
Rate of compensation increase (as a percent)	8.00%	8.00%	8.00%
Pre-tax decrease to accumulated other comprehensive income	$ 788,876	$ (154,986)	$ (547,569)
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	$ 13,100	$ 17,500	$ 15,400
Other Liabilities
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	56,400	59,700	80,400
Retirement Savings Account
Defined Benefit Plan Disclosure [Line Items]
Costs recognized	5,700	5,300	4,800
Tax-Deferred Savings Plans
Defined Benefit Plan Disclosure [Line Items]
Costs recognized	8,200	7,800	7,200
Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Net Periodic Benefit Cost (Income)	(4,622)	23,087	24,911
Accumulated Benefit Obligation	751,912	793,555	1,060,659
Projected benefit obligation	$ 768,750	$ 813,828	$ 1,098,456	$ 1,139,105
Discount rate (as a percent)	5.99%	5.57%	2.75%
Rate of compensation increase (as a percent)	4.60%	4.60%	4.70%
Expected future benefit payments in 2024	$ 67,900
Expected future benefit payments in 2025	67,400
Expected future benefit payments in 2026	66,900
Expected future benefit payments in 2027	66,200
Expected future benefit payments 2028	65,500
Expected future benefit payments 2029 through 2033	310,400
Employer contributions	$ 0	$ 20,400	$ 20,000
Expected long term rate of return on plan assets (as a percent)	6.90%	5.20%	6.00%
Retirement Plan | Effective Fiscal 2024
Defined Benefit Plan Disclosure [Line Items]
Expected long term rate of return on plan assets (as a percent)	7.40%
Retirement Plan | Minimum
Defined Benefit Plan Disclosure [Line Items]
Annual contribution expected for next fiscal year	$ 0
Retirement Plan | Maximum
Defined Benefit Plan Disclosure [Line Items]
Annual contribution expected for next fiscal year	5,000
Retirement Plan | Other Actuarial Experience
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	1,800
Retirement Plan | Discount Rate Change
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	(28,400)	$ (262,200)	$ (11,200)
Retirement Plan | Mortality Improvement Projection Scale
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	(700)
Non-Qualified Benefit Plans, Other Post-Retirement Benefit Plan and Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Increase to other regulatory assets	28,700
Pre-tax decrease to accumulated other comprehensive income	8,000
Other Post-Retirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net Periodic Benefit Cost (Income)	(3,404)	(5,664)	10,371
Projected benefit obligation	$ 274,278	$ 299,283	$ 431,213	$ 476,722
Discount rate (as a percent)	5.99%	5.56%	2.76%
Rate of compensation increase (as a percent)	4.60%	4.60%	4.70%
Expected future benefit payments in 2024	$ 25,334
Expected future benefit payments in 2025	25,479
Expected future benefit payments in 2026	25,466
Expected future benefit payments in 2027	25,389
Expected future benefit payments 2028	25,260
Expected future benefit payments 2029 through 2033	120,390
Employer contributions	$ 235	$ 3,082	$ 3,068
Expected long term rate of return on plan assets (as a percent)	5.70%	5.20%	5.40%
Other Post-Retirement Benefits | Effective Fiscal 2024
Defined Benefit Plan Disclosure [Line Items]
Expected long term rate of return on plan assets (as a percent)	6.00%
Other Post-Retirement Benefits | Other Actuarial Experience
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	$ (12,900)	$ (22,500)	$ (26,600)
Other Post-Retirement Benefits | Discount Rate Change
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	(10,700)	(98,900)	(2,500)
Other Post-Retirement Benefits | Mortality Improvement Projection Scale
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	(400)	$ 1,100	(2,000)
Other Post-Retirement Benefits | Health Care Cost Trend Rates
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	3,200		$ (3,700)
Other Than VEBA Trust And 401(h) Accounts
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 200