Retirement Plan and Other Post-Retirement Benefits - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost (income)	$ 4,600	$ 9,500	$ 8,300
Accumulated Benefit Obligation	50,900	57,500	58,500
Projected benefit obligation	$ 58,400	$ 66,000	$ 69,500
Discount rate (as a percent)	4.86%	4.71%	5.91%
Rate of compensation increase (as a percent)	8.00%	8.00%	8.00%
Pre-tax decrease to accumulated other comprehensive income	$ (59,398)	$ 55,235	$ 788,876
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	$ 11,600	$ 14,100	$ 13,100
Other Liabilities
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	46,800	51,900	56,400
Retirement Savings Account
Defined Benefit Plan Disclosure [Line Items]
Costs recognized	7,100	6,500	5,700
Tax-Deferred Savings Plans
Defined Benefit Plan Disclosure [Line Items]
Costs recognized	9,800	9,000	8,200
Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost (income)	(12,195)	(5,334)	(4,622)
Accumulated Benefit Obligation	765,747	804,916	751,912
Projected benefit obligation	$ 779,663	$ 822,306	$ 768,750	$ 813,828
Discount rate (as a percent)	5.28%	4.97%	5.99%
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 2026	67,600
Expected future benefit payments in 2027	66,900
Expected future benefit payments in 2028	66,100
Expected future benefit payments in 2029	65,100
Expected future benefit payments 2030	64,000
Expected future benefit payments 2031 through 2035	297,800
Employer contributions	$ 0	$ 0	$ 0
Expected long term rate of return on plan assets (as a percent)	6.60%	7.40%	6.90%
Retirement Plan | Scenario, Plan
Defined Benefit Plan Disclosure [Line Items]
Expected long term rate of return on plan assets (as a percent)	7.10%
Retirement Plan | Other Actuarial Experience
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	$ 4,200
Retirement Plan | Discount Rate Change
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	(21,400)	$ 69,600	$ (28,400)
Non-Qualified Benefit Plans, Other Post-Retirement Benefit Plan and Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Increase to other regulatory assets	23,800
Pre-tax decrease to accumulated other comprehensive income	10,300
Other Post-Retirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost (income)	(15,373)	(4,583)	(3,404)
Projected benefit obligation	$ 330,584	$ 321,025	$ 274,278	$ 299,283
Discount rate (as a percent)	5.30%	4.98%	5.99%
Rate of compensation increase (as a percent)	4.60%	4.60%	4.60%
Expected future benefit payments in 2026	$ 26,899
Expected future benefit payments in 2027	27,556
Expected future benefit payments in 2028	28,060
Expected future benefit payments in 2029	28,344
Expected future benefit payments 2030	28,406
Expected future benefit payments 2031 through 2035	135,297
Employer contributions	$ 594	$ 479	$ 235
Expected long term rate of return on plan assets (as a percent)	5.40%	6.00%	5.70%
Other Post-Retirement Benefits | Scenario, Plan
Defined Benefit Plan Disclosure [Line Items]
Expected long term rate of return on plan assets (as a percent)	6.10%
Other Post-Retirement Benefits | Other Actuarial Experience
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	$ 8,800	$ 1,200	$ (12,900)
Other Post-Retirement Benefits | Discount Rate Change
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	(9,900)	28,100	(10,700)
Other Post-Retirement Benefits | Mortality Improvement Projection Scale
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation			(400)
Other Post-Retirement Benefits | Health Care Cost Trend Rates
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	18,200	$ 25,200	$ 3,200
Other Than VEBA Trust And 401(h) Accounts
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	600
VEBA Trust
Defined Benefit Plan Disclosure [Line Items]
Annual contribution expected for next fiscal year	0
Employer contributions	$ 0