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Financial Instruments - Narrative (Details)
3 Months Ended 6 Months Ended
Mar. 31, 2020
USD ($)
Mar. 31, 2019
USD ($)
Mar. 31, 2020
USD ($)
Bcf
Mar. 31, 2019
USD ($)
Sep. 30, 2021
Apr. 01, 2020
USD ($)
Sep. 30, 2019
USD ($)
Derivative [Line Items]              
Net realized gain (loss) in AOCI $ (112,600,000)   $ (112,600,000)        
Contract Netting 0   0       $ 0
Cash collateral 0   0       $ 0
Net gain (loss) on settled interest rate agreements $ (1,400,000) $ (600,000) $ (2,700,000) $ (1,200,000)      
Gas Purchases | Not Designated as Hedging Instrument | Commodity contracts              
Derivative [Line Items]              
Hedging percent 49.00%   49.00%        
Energy measure | Bcf     19.9        
Gas Purchases | Not Designated as Hedging Instrument | Commodity contracts | Long              
Derivative [Line Items]              
Energy measure | Bcf     4.51        
Minimum | Gas Purchases | Not Designated as Hedging Instrument | Commodity contracts              
Derivative [Line Items]              
Hedging percent 25.00%   25.00%        
Maximum | Gas Purchases | Not Designated as Hedging Instrument | Commodity contracts              
Derivative [Line Items]              
Hedging percent 50.00%   50.00%        
Subsequent Event | Designated as Hedging Instrument | Forward Interest Rate Swap 1              
Derivative [Line Items]              
Derivative, notional amount           $ 500,000,000  
Subsequent Event | Designated as Hedging Instrument | Forward Interest Rate Swap 2              
Derivative [Line Items]              
Derivative, notional amount           $ 450,000,000  
Forecast | Unsecured Senior Notes In Fiscal 2021              
Derivative [Line Items]              
Interest rate         0.69%    
Forecast | Unsecured Senior Notes In Fiscal 2022              
Derivative [Line Items]              
Interest rate         1.33%