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Financial Derivative Instruments - Fair Values of Fuel Derivatives Amounts Posted as Collateral (Details)
$ in Millions
Sep. 30, 2023
USD ($)
If credit rating is investment grade, fair value of fuel derivative level at which:  
Letter of credit substitution percentage 100.00%
Cash collateral percentage 100.00%
Fuel derivative contracts  
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]  
Fair value of fuel derivatives $ 487
Cash collateral held from CP 100
Fuel derivative contracts | Counterparty A  
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]  
Fair value of fuel derivatives 120
Cash collateral held from CP 100
Fuel derivative contracts | Counterparty A | Minimum  
If credit rating is investment grade, fair value of fuel derivative level at which:  
Cash is provided to CP (100)
Cash is received from CP 0
Fuel derivative contracts | Counterparty B  
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]  
Fair value of fuel derivatives 57
Cash collateral held from CP 0
Fuel derivative contracts | Counterparty B | Minimum  
If credit rating is investment grade, fair value of fuel derivative level at which:  
Cash is provided to CP (50)
Cash is received from CP 150
Fuel derivative contracts | Counterparty C  
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]  
Fair value of fuel derivatives 84
Cash collateral held from CP 0
Fuel derivative contracts | Counterparty C | Minimum  
If credit rating is investment grade, fair value of fuel derivative level at which:  
Cash is provided to CP (75)
Cash is received from CP 250
Fuel derivative contracts | Counterparty D  
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]  
Fair value of fuel derivatives 28
Cash collateral held from CP 0
Fuel derivative contracts | Counterparty D | Minimum  
If credit rating is investment grade, fair value of fuel derivative level at which:  
Cash is provided to CP (125)
Cash is received from CP 125
Fuel derivative contracts | Counterparty E  
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]  
Fair value of fuel derivatives 77
Cash collateral held from CP 0
Fuel derivative contracts | Counterparty E | Minimum  
If credit rating is investment grade, fair value of fuel derivative level at which:  
Cash is provided to CP (40)
Cash is received from CP 100
Fuel derivative contracts | Counterparty F  
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]  
Fair value of fuel derivatives 35
Cash collateral held from CP 0
Fuel derivative contracts | Counterparty F | Minimum  
If credit rating is investment grade, fair value of fuel derivative level at which:  
Cash is provided to CP (65)
Cash is received from CP 70
Fuel derivative contracts | Counterparty G  
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]  
Fair value of fuel derivatives 60
Cash collateral held from CP 0
Fuel derivative contracts | Counterparty G | Minimum  
If credit rating is investment grade, fair value of fuel derivative level at which:  
Cash is provided to CP (100)
Cash is received from CP 100
Fuel derivative contracts | Counterparty H  
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]  
Fair value of fuel derivatives 26
Cash collateral held from CP 0
Fuel derivative contracts | Counterparty H | Minimum  
If credit rating is investment grade, fair value of fuel derivative level at which:  
Cash is provided to CP (100)
Cash is received from CP $ 100