Financial Instruments - Narrative (Details)	3 Months Ended
	Dec. 31, 2025 USD ($) MMcf	Dec. 31, 2024 USD ($)	Sep. 30, 2025 USD ($)
Derivative [Line Items]
Purchase commitment volume | MMcf	19,816
Contract netting	$ 0		$ 0
Cash collateral	0		$ 0
Net (gain) loss on settled interest rate agreements	(6,100,000)	$ (5,100,000)
Net realized gains in AOCI	$ 470,000,000.0
Designated As Hedge | Gas Purchases | Commodity contracts
Derivative [Line Items]
Purchase commitment volume | MMcf	23,800
Designated As Hedge | Gas Purchases | Commodity contracts | Minimum
Derivative [Line Items]
Hedging percent	25.00%
Designated As Hedge | Gas Purchases | Commodity contracts | Maximum
Derivative [Line Items]
Hedging percent	50.00%