Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Statement of Comprehensive Income [Abstract]
Net Income Available for Common Stock	$ 77,513	$ 476,866	$ 566,021
Other Comprehensive Income (Loss), Before Tax:
Increase (Decrease) in the Funded Status of the Pension and Other Post-Retirement Benefit Plans	(17,511)	(9,660)	9,561
Reclassification Adjustment for Amortization of Prior Year Funded Status of the Pension and Other Post-Retirement Benefit Plans	2,507	1,674	11,054
Unrealized Gain (Loss) on Derivative Financial Instruments Arising During the Period	286,894	708,206	(1,050,831)
Reclassification Adjustment for Realized (Gains) Losses on Derivative Financial Instruments in Net Income	(216,655)	88,656	882,581
Other Post-Retirement Adjustment for Regulatory Proceeding	0	0	(7,351)
Other Comprehensive Income (Loss), Before Tax	55,235	788,876	(154,986)
Income Tax Expense (Benefit) Related to the Increase (Decrease) in the Funded Status of the Pension and Other Post-Retirement Benefit Plans	(3,996)	(2,284)	2,169
Reclassification Adjustment for Income Tax Benefit Related to the Amortization of the Prior Year Funded Status of the Pension and Other Post-Retirement Benefit Plans	584	411	2,574
Income Tax Expense (Benefit) Related to Unrealized Gain (Loss) on Derivative Financial Instruments Arising During the Period	81,197	214,270	(287,608)
Reclassification Adjustment for Income Tax Benefit (Expense) on Realized Losses (Gains) from Derivative Financial Instruments in Net Income	(62,134)	5,806	241,559
Income Tax Expense (Benefit) Related to Other Post-Retirement Adjustment for Regulatory Proceeding	0	0	(1,544)
Income Taxes — Net	15,651	218,203	(42,850)
Other Comprehensive Income (Loss)	39,584	570,673	(112,136)
Comprehensive Income	$ 117,097	$ 1,047,539	$ 453,885