Summary of Significant Accounting Policies - Components of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Other Comprehensive Gains and Losses Before Reclassifications	$ (13,456)	$ 192,182
Amounts Reclassified From Other Comprehensive Loss	(30,290)	(152,598)
Gains and Losses on Derivative Financial Instruments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	55,799	4,623
Other Comprehensive Gains and Losses Before Reclassifications	(2,308)	205,697
Amounts Reclassified From Other Comprehensive Loss	(33,541)	(154,521)
Ending balance	19,950	55,799
Funded Status of the Pension and Other Post-Retirement Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(71,275)	(59,683)
Other Comprehensive Gains and Losses Before Reclassifications	(11,148)	(13,515)
Amounts Reclassified From Other Comprehensive Loss	3,251	1,923
Ending balance	(79,172)	(71,275)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(15,476)	(55,060)
Ending balance	$ (59,222)	$ (15,476)