Income Taxes - Significant Components of Deferred Tax Liabilities and Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Sep. 30, 2022
Deferred Tax Liabilities:
Unrealized Hedging Gains	$ 3,385	$ 0
Property, Plant and Equipment	1,178,893	954,757
Pension and Other Post-Retirement Benefit Costs	44,358	30,132
Other	21,470	48,893
Total Deferred Tax Liabilities	1,248,106	1,033,782
Deferred Tax Assets:
Unrealized Hedging Losses	0	(215,187)
Tax Loss and Credit Carryforwards	(33,744)	(50,686)
Pension and Other Post-Retirement Benefit Costs	(41,843)	(37,250)
Other	(48,349)	(32,430)
Total Deferred Tax Assets	(123,936)	(335,553)
Total Net Deferred Income Taxes	$ 1,124,170	$ 698,229