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Financial Derivative Instruments - Fair Values of Fuel Derivatives Amounts Posted as Collateral (Details) $ in Millions | Mar. 31, 2021 USD ($)
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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.

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