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Financial Instruments - Narrative (Details)
3 Months Ended 6 Months Ended 9 Months Ended
Jun. 30, 2020
USD ($)
Jun. 30, 2019
USD ($)
Mar. 31, 2020
Bcf
Jun. 30, 2020
USD ($)
Bcf
Jun. 30, 2019
USD ($)
Sep. 30, 2025
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2021
Jul. 31, 2020
USD ($)
Sep. 30, 2019
USD ($)
Derivative [Line Items]                      
Net realized gain (loss) in AOCI $ (112,100,000)     $ (112,100,000)              
Contract Netting 0     0             $ 0
Cash collateral 0     0             $ 0
Net gain (loss) on settled interest rate agreements (1,400,000) $ (1,400,000)   (4,100,000) $ (2,600,000)            
Designated as Hedging Instrument | Forward Interest Rate Swap 1                      
Derivative [Line Items]                      
Derivative, notional amount 500,000,000     500,000,000              
Designated as Hedging Instrument | Forward Interest Rate Swap 2                      
Derivative [Line Items]                      
Derivative, notional amount $ 450,000,000     $ 450,000,000              
Gas Purchases | Not Designated as Hedging Instrument | Commodity contracts                      
Derivative [Line Items]                      
Hedging percent     49.00%                
Energy measure | Bcf     19.9                
Gas Purchases | Not Designated as Hedging Instrument | Commodity contracts | Long                      
Derivative [Line Items]                      
Energy measure | Bcf       10.05              
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%              
Forecast | Unsecured Senior Notes In Fiscal 2023                      
Derivative [Line Items]                      
Interest rate             1.36%        
Forecast | Unsecured Senior Notes In Fiscal 2025                      
Derivative [Line Items]                      
Interest rate           1.35%          
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%      
Subsequent Event | Designated as Hedging Instrument | Forward Interest Rate Swap 3                      
Derivative [Line Items]                      
Derivative, notional amount                   $ 300,000,000  
Subsequent Event | Designated as Hedging Instrument | Forward Interest Rate Swap 4                      
Derivative [Line Items]                      
Derivative, notional amount                   $ 300,000,000