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Financial Instruments - Narrative (Details)
1 Months Ended 3 Months Ended 12 Months Ended
Sep. 30, 2021
USD ($)
Sep. 30, 2021
USD ($)
MMcf
Sep. 30, 2021
USD ($)
$ / MMcf
Bcf
Sep. 30, 2020
USD ($)
Sep. 30, 2019
USD ($)
Derivative [Line Items]          
Net loss on settled treasury lock agreements reclassified from AOCI into interest expense   $ (5,900,000)   $ 5,500,000 $ 3,900,000
AOCI impact from settlement of financial instruments $ (61,700,000) $ (61,700,000) $ (61,700,000)    
Weighted Average          
Derivative [Line Items]          
Derivative flow rate | $ / MMcf     2.86    
Commodity contracts          
Derivative [Line Items]          
Purchase commitment volume | MMcf   23,737      
Unsecured 2.85% Senior Notes, due 2052          
Derivative [Line Items]          
Debt face amount $ 600,000,000 $ 600,000,000 $ 600,000,000    
Not Designated as Hedging Instrument | Gas Purchases | Commodity contracts          
Derivative [Line Items]          
Hedging percent 39.00% 39.00% 39.00%    
Purchase commitment volume | Bcf     15.8    
Not Designated as Hedging Instrument | Gas Purchases | Commodity contracts | Minimum          
Derivative [Line Items]          
Hedging percent 25.00% 25.00% 25.00%    
Not Designated as Hedging Instrument | Gas Purchases | Commodity contracts | Maximum          
Derivative [Line Items]          
Hedging percent 50.00% 50.00% 50.00%    
Designated as Hedging Instrument | Forward Interest Rate Swap 1          
Derivative [Line Items]          
Derivative notional amount $ 1,400,000,000 $ 1,400,000,000 $ 1,400,000,000    
Settlement of interest rate swaps $ 62,200,000