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Financial Instruments - Narrative (Details)
3 Months Ended 6 Months Ended
Mar. 31, 2025
USD ($)
Mar. 31, 2024
USD ($)
Mar. 31, 2025
USD ($)
MMcf
Mar. 31, 2024
USD ($)
Sep. 30, 2024
USD ($)
Derivative [Line Items]          
Purchase commitment volume | MMcf     4,017    
Contract netting $ 0   $ 0   $ 0
Cash collateral 0   0   $ 0
Net (gain) loss on settled interest rate agreements (5,100,000) $ (3,200,000) (10,200,000) $ (6,400,000)  
Net realized gains in AOCI $ 387,000,000.0   $ 387,000,000.0    
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%