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Financial Derivative Instruments - Fair Values of Fuel Derivatives Amounts Posted as Collateral (Details)
$ in Millions
Mar. 31, 2021
USD ($)
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]  
Letter of Credit Percent of Collateral 100.00%
Cash Collateral Percent Of Fair Value Fuel Derivatives Contracts 100.00%
Fuel derivatives  
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]  
Derivative Asset, Fair Value, Amount Not Offset Against Collateral $ 249
Cash collateral held (from) by CP 72
Counterparty A | Fuel derivatives  
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]  
Derivative Asset, Fair Value, Amount Not Offset Against Collateral 60
Cash collateral held (from) by CP 72
Counterparty B | Fuel derivatives  
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]  
Derivative Asset, Fair Value, Amount Not Offset Against Collateral 28
Cash collateral held (from) by CP 0
Counterparty C | Fuel derivatives  
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]  
Derivative Asset, Fair Value, Amount Not Offset Against Collateral 60
Cash collateral held (from) by CP 0
Counterparty D | Fuel derivatives  
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]  
Derivative Asset, Fair Value, Amount Not Offset Against Collateral 28
Cash collateral held (from) by CP 0
Counterparty E | Fuel derivatives  
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]  
Derivative Asset, Fair Value, Amount Not Offset Against Collateral 30
Cash collateral held (from) by CP 0
Counterparty F | Fuel derivatives  
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]  
Derivative Asset, Fair Value, Amount Not Offset Against Collateral 21
Cash collateral held (from) by CP 0
Counterparty G | Fuel derivatives  
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]  
Derivative Asset, Fair Value, Amount Not Offset Against Collateral 18
Cash collateral held (from) by CP 0
Counterparty Other | Fuel derivatives  
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]  
Derivative Asset, Fair Value, Amount Not Offset Against Collateral 4 [1]
Cash collateral held (from) by CP 0 [1]
Minimum | Counterparty A | Fuel derivatives  
If credit rating is investment grade, fair value of fuel derivative level at which:  
Fair value of fuel derivative levels at which cash is provided to CP Threshold 1 (100)
Fair value of fuel derivative level at which cash is received from CP 0 [2]
If credit rating is non-investment grade, fair value of fuel derivative level at which:  
Fair value of fuel derivative level at which cash is received from CP [3]
Minimum | Counterparty B | Fuel derivatives  
If credit rating is investment grade, fair value of fuel derivative level at which:  
Fair value of fuel derivative levels at which cash is provided to CP Threshold 1 (50)
Fair value of fuel derivative level at which cash is received from CP 150 [2]
If credit rating is non-investment grade, fair value of fuel derivative level at which:  
Fair value of fuel derivative level at which cash is received from CP [3]
Minimum | Counterparty C | Fuel derivatives  
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]  
Option to substitute LC for cash Threshold 1 [4]
If credit rating is investment grade, fair value of fuel derivative level at which:  
Fair value of fuel derivative levels at which cash is provided to CP Threshold 1 (75)
Fair value of fuel derivative level at which cash is received from CP 250 [2]
If credit rating is non-investment grade, fair value of fuel derivative level at which:  
Fair value of fuel derivative level at which cash is received from CP [3]
Minimum | Counterparty D | Fuel derivatives  
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]  
Option to substitute LC for cash Threshold 1 [4]
If credit rating is investment grade, fair value of fuel derivative level at which:  
Fair value of fuel derivative levels at which cash is provided to CP Threshold 1 (125)
Fair value of fuel derivative level at which cash is received from CP 125 [2]
If credit rating is non-investment grade, fair value of fuel derivative level at which:  
Fair value of fuel derivative level at which cash is received from CP [3]
Minimum | Counterparty E | Fuel derivatives  
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]  
Option to substitute LC for cash Threshold 1 [4]
If credit rating is investment grade, fair value of fuel derivative level at which:  
Fair value of fuel derivative levels at which cash is provided to CP Threshold 1 (40)
Fair value of fuel derivative level at which cash is received from CP 100 [2]
If credit rating is non-investment grade, fair value of fuel derivative level at which:  
Fair value of fuel derivative level at which cash is received from CP [3]
Minimum | Counterparty F | Fuel derivatives  
If credit rating is investment grade, fair value of fuel derivative level at which:  
Fair value of fuel derivative levels at which cash is provided to CP Threshold 1 (65)
Fair value of fuel derivative level at which cash is received from CP 70 [2]
If credit rating is non-investment grade, fair value of fuel derivative level at which:  
Fair value of fuel derivative level at which cash is received from CP [3]
Minimum | Counterparty G | Fuel derivatives  
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]  
Option to substitute LC for cash Threshold 1 [4]
If credit rating is investment grade, fair value of fuel derivative level at which:  
Fair value of fuel derivative levels at which cash is provided to CP Threshold 1 (100)
Fair value of fuel derivative level at which cash is received from CP 100 [2]
If credit rating is non-investment grade, fair value of fuel derivative level at which:  
Fair value of fuel derivative level at which cash is received from CP [3]
[1] Individual counterparties with fair value of fuel derivatives < $4 million.
[2] Thresholds may vary based on changes in credit ratings within investment grade.
[3] Cash collateral is provided at 100 percent of fair value of fuel derivative contracts
[4] The Company has the option to substitute letters of credit for 100 percent of cash collateral requirement.