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Financial Instruments - Narrative (Details)	12 Months Ended
	Sep. 30, 2025 USD ($) $ / Mcf MMcf	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)
Derivative [Line Items]
Purchase commitment volume | MMcf	32,566
Contract netting	$ 0	$ 0
Cash collateral	0	0
Net (gain) loss on settled interest rate agreements	(20,500,000)	$ (12,800,000)	$ (2,700,000)
Net realized gains in AOCI	$ 474,800,000
Not Designated As Hedges | Gas Purchases | Commodity contracts
Derivative [Line Items]
Purchase commitment volume | MMcf	24,000.0
Not Designated As Hedges | Gas Purchases | Commodity contracts | Minimum
Derivative [Line Items]
Hedging percent	25.00%
Not Designated As Hedges | Gas Purchases | Commodity contracts | Maximum
Derivative [Line Items]
Hedging percent	50.00%
Not Designated As Hedges | Gas Purchases | Commodity contracts | Weighted Average
Derivative [Line Items]
Derivative flow rate (USD per Mcf) | $ / Mcf	3.83

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