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Financial Instruments - Narrative (Details)
3 Months Ended 6 Months Ended
Mar. 31, 2023
USD ($)
Mar. 31, 2022
USD ($)
Mar. 31, 2023
USD ($)
MMcf
Mar. 31, 2022
USD ($)
Apr. 30, 2023
USD ($)
Sep. 30, 2022
USD ($)
Mar. 31, 2021
USD ($)
Derivative [Line Items]              
Energy measure | MMcf     4,123        
Contract netting $ 0   $ 0     $ 0  
Cash collateral 0   0     $ 0  
Net (gain) loss on settled interest rate agreements (700,000) $ 1,000,000 (1,400,000) $ 1,900,000      
Net realized gains in AOCI 93,100,000   93,100,000        
Senior Notes              
Derivative [Line Items]              
Debt instrument, face amount             $ 2,200,000,000
Forward Interest Rate Swap | Senior Notes              
Derivative [Line Items]              
Debt instrument, face amount $ 150,000,000   $ 150,000,000        
Forward Interest Rate Swap | Senior Notes | Subsequent Event              
Derivative [Line Items]              
Debt instrument, face amount         $ 100,000,000    
Gas Purchases | Not Designated As Hedges: | Commodity contracts              
Derivative [Line Items]              
Hedging percent 32.00%   32.00%        
Energy measure | MMcf     17,700        
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%