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Financial Instruments - Narrative (Details)
12 Months Ended
Sep. 30, 2023
USD ($)
$ / Mcf
MMcf
Sep. 30, 2022
USD ($)
Sep. 30, 2021
USD ($)
Derivative [Line Items]      
Purchase commitment volume | MMcf 31,289    
Contract netting $ 0 $ 0  
Cash collateral 0 0  
Net (gain) loss on settled interest rate agreements (2,700,000) $ 3,800,000 $ 5,900,000
Net realized gains in AOCI $ 224,800,000    
Not Designated As Hedges: | Gas Purchases | Commodity contracts      
Derivative [Line Items]      
Purchase commitment volume | MMcf 17,300    
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 5.28