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Financial Instruments - Narrative (Details)
3 Months Ended 12 Months Ended
Dec. 31, 2020
USD ($)
Bcf
Dec. 31, 2019
USD ($)
Sep. 30, 2020
USD ($)
Bcf
Derivative [Line Items]      
Contract netting $ 0 $ 0 $ 0
Cash collateral 0 0  
Net gain (loss) on settled interest rate agreements (1,500,000) $ (1,400,000)  
Net realized gain (loss) in AOCI $ (113,400,000)    
Gas Purchases | Not Designated as Hedging Instrument | Commodity contracts      
Derivative [Line Items]      
Hedging percent     39.00%
Energy measure | Bcf     15.8
Gas Purchases | Not Designated as Hedging Instrument | Commodity contracts | Long      
Derivative [Line Items]      
Energy measure | Bcf 11.64    
Minimum | Gas Purchases | Not Designated as Hedging Instrument | Commodity contracts      
Derivative [Line Items]      
Hedging percent 25.00%    
Maximum | Gas Purchases | Not Designated as Hedging Instrument | Commodity contracts      
Derivative [Line Items]      
Hedging percent 50.00%