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Financial Instruments - Narrative (Details)
3 Months Ended 9 Months Ended
Jun. 30, 2024
USD ($)
Jun. 30, 2023
USD ($)
Jun. 30, 2024
USD ($)
MMcf
Jun. 30, 2023
USD ($)
Sep. 30, 2023
USD ($)
Derivative [Line Items]          
Purchase commitment volume | MMcf     15,455    
Contract netting $ 0   $ 0   $ 0
Cash collateral 0   0   $ 0
Net (gain) loss on settled interest rate agreements (3,200,000) $ (700,000) (9,600,000) $ (2,100,000)  
Net realized gains in AOCI $ 217,300,000   $ 217,300,000    
Not Designated as Hedging Instrument | Gas Purchases | Commodity contracts          
Derivative [Line Items]          
Purchase commitment volume | MMcf     27,600    
Not Designated as Hedging Instrument | Gas Purchases | Commodity contracts | Minimum          
Derivative [Line Items]          
Hedging percent 25.00%   25.00%    
Not Designated as Hedging Instrument | Gas Purchases | Commodity contracts | Maximum          
Derivative [Line Items]          
Hedging percent 50.00%   50.00%