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Financial Instruments - Narrative (Details)
3 Months Ended
Dec. 31, 2021
USD ($)
Bcf
MMcf
Dec. 31, 2020
USD ($)
Sep. 30, 2021
USD ($)
Derivative [Line Items]      
Energy measure | MMcf 12,982    
Contract netting $ 0   $ 0
Cash collateral 0   $ 0
Net gain (loss) on settled interest rate agreements (1,000,000) $ (1,500,000)  
Net realized gain (loss) in AOCI $ (61,000,000)    
Gas Purchases | Not Designated As Hedges: | Commodity contracts      
Derivative [Line Items]      
Hedging percent 42.00%    
Energy measure | Bcf 23.9    
Minimum | Gas Purchases | Not Designated As Hedges: | Commodity contracts      
Derivative [Line Items]      
Hedging percent 25.00%    
Maximum | Gas Purchases | Not Designated As Hedges: | Commodity contracts      
Derivative [Line Items]      
Hedging percent 50.00%