Financial Instruments - Narrative (Details)	3 Months Ended		9 Months Ended
	Jun. 30, 2025 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2025 USD ($) MMcf	Jun. 30, 2024 USD ($)	Sep. 30, 2024 USD ($)
Derivative [Line Items]
Purchase commitment volume | MMcf			14,909
Contract netting	$ 0		$ 0		$ 0
Cash collateral	0		0		$ 0
Net (gain) loss on settled interest rate agreements	(5,100,000)	$ (3,200,000)	(15,400,000)	$ (9,600,000)
Net realized gains in AOCI	$ 383,000,000.0		$ 383,000,000.0
Designated As Hedge | Gas Purchases | Commodity contracts
Derivative [Line Items]
Purchase commitment volume | MMcf			24,000.0
Designated As Hedge | Gas Purchases | Commodity contracts | Minimum
Derivative [Line Items]
Hedging percent	25.00%		25.00%
Designated As Hedge | Gas Purchases | Commodity contracts | Maximum
Derivative [Line Items]
Hedging percent	50.00%		50.00%