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Financial Instruments - Narrative (Details)
3 Months Ended 9 Months Ended
Jun. 30, 2021
USD ($)
Jun. 30, 2020
USD ($)
Jun. 30, 2021
USD ($)
Bcf
Jun. 30, 2020
USD ($)
Jul. 31, 2021
USD ($)
Sep. 30, 2020
USD ($)
Derivative [Line Items]            
Contract netting $ 0   $ 0     $ 0
Cash collateral 0   0     $ 0
Net gain (loss) on settled interest rate agreements (1,500,000) $ (1,400,000) (4,400,000) $ (4,100,000)    
Net realized gain (loss) in AOCI (111,100,000)   (111,100,000)      
Designated As Hedges: | Forward Interest Rate Swap 1            
Derivative [Line Items]            
Amount Hedged $ 525,000,000   $ 525,000,000      
Designated As Hedges: | Forward Interest Rate Swap 1 | Subsequent Event            
Derivative [Line Items]            
Amount Hedged         $ 875,000,000  
Gas Purchases | Not Designated As Hedges: | Commodity contracts            
Derivative [Line Items]            
Hedging percent 39.00%   39.00%      
Energy measure | Bcf     15.8      
Gas Purchases | Not Designated As Hedges: | Commodity contracts | Long            
Derivative [Line Items]            
Energy measure | Bcf     16.80      
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%