Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Statement of Comprehensive Income [Abstract]
Net Income Available for Common Stock	$ 476,866	$ 566,021	$ 363,647
Other Comprehensive Income (Loss), Before Tax:
Increase (Decrease) in the Funded Status of the Pension and Other Post-Retirement Benefit Plans	(9,660)	9,561	17,862
Reclassification Adjustment for Amortization of Prior Year Funded Status of the Pension and Other Post-Retirement Benefit Plans	1,674	11,054	16,229
Unrealized Gain (Loss) on Derivative Financial Instruments Arising During the Period	708,206	(1,050,831)	(665,371)
Reclassification Adjustment for Realized (Gains) Losses on Derivative Financial Instruments in Net Income	88,656	882,581	83,711
Other Post-Retirement Adjustment for Regulatory Proceeding	0	(7,351)	0
Other Comprehensive Income (Loss), Before Tax	788,876	(154,986)	(547,569)
Income Tax Expense (Benefit) Related to the Increase (Decrease) in the Funded Status of the Pension and Other Post-Retirement Benefit Plans	(2,284)	2,169	4,072
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	411	2,574	3,762
Income Tax Expense (Benefit) Related to Unrealized Gain (Loss) on Derivative Financial Instruments Arising During the Period	214,270	(287,608)	(179,028)
Reclassification Adjustment for Income Tax Benefit (Expense) on Realized Losses (Gains) from Derivative Financial Instruments in Net Income	5,806	241,559	22,465
Income Tax Expense (Benefit) Related to Other Post-Retirement Adjustment for Regulatory Proceeding	0	(1,544)	0
Income Taxes — Net	218,203	(42,850)	(148,729)
Other Comprehensive Income (Loss)	570,673	(112,136)	(398,840)
Comprehensive Income (Loss)	$ 1,047,539	$ 453,885	$ (35,193)