Income Taxes - Significant Components of Deferred Tax Liabilities and Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Sep. 30, 2024
Deferred Tax Liabilities:
Unrealized Hedging Gains	$ 9,036	$ 22,433
Property, Plant and Equipment	1,222,869	1,134,727
Pension and Other Post-Retirement Benefit Costs	72,691	59,877
Other	24,531	13,885
Total Deferred Tax Liabilities	1,329,127	1,230,922
Deferred Tax Assets:
Tax Loss and Credit Carryforwards	(28,364)	(31,111)
Pension and Other Post-Retirement Benefit Costs	(54,071)	(49,622)
Other	(21,430)	(39,024)
Total Deferred Tax Assets	(103,865)	(119,757)
Total Net Deferred Income Taxes	$ 1,225,262	$ 1,111,165