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Financial Instruments - Narrative (Details)	3 Months Ended		9 Months Ended
	Jun. 30, 2022 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2022 USD ($) Bcf	Jun. 30, 2021 USD ($)	Sep. 30, 2021 USD ($)
Derivative [Line Items]
Energy measure | Bcf			5,819,000,000
Contract netting	$ 0		$ 0		$ 0
Cash collateral	0		0		$ 0
Net gain (loss) on settled interest rate agreements	(1,000,000)	$ (1,500,000)	(2,900,000)	$ (4,400,000)
Net realized gain (loss) in AOCI	$ (59,500,000)		$ (59,500,000)
Gas Purchases | Not Designated As Hedges: | Commodity contracts
Derivative [Line Items]
Hedging percent	42.00%		42.00%
Energy measure | Bcf			23.9
Minimum | Gas Purchases | Not Designated As Hedges: | Commodity contracts
Derivative [Line Items]
Hedging percent	25.00%		25.00%
Maximum | Gas Purchases | Not Designated As Hedges: | Commodity contracts
Derivative [Line Items]
Hedging percent	50.00%		50.00%

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