Financial Derivative Instruments - Fair Values of Fuel Derivatives Amounts Posted as Collateral (Details) $ in Millions	Dec. 31, 2020 USD ($)
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]
Maximum Sum Of Derivatives Of Counterparty To Be Included In Other	$ 10
If credit rating is non-investment grade, fair value of fuel derivative level at which:
Cash Collateral Percent Of Fair Value Fuel Derivatives Contracts	100.00%
Letter of Credit Percent of Collateral	100.00%
Fuel derivatives
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]
Fair value of fuel derivatives - asset	$ 134
Collateral Already Posted, Aggregate Fair Value	34
Letters of credit (LC)	0
Counterparty A | Fuel derivatives
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]
Fair value of fuel derivatives - asset	33
Collateral Already Posted, Aggregate Fair Value	34
Letters of credit (LC)	0
Counterparty A | Minimum | 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	[1]
Fair value of fuel derivative levels at which cash is pledged to CP	(200)	[2]
If credit rating is non-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	0
Fair value of fuel derivative levels at which cash is provided to CP Threshold 2	(600)
Fair value of fuel derivative level at which cash is received from CP		[3]
Fair value of fuel derivative level at which cash is pledged to CP	(200)
Counterparty A | Maximum | 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 pledged to CP	(600)	[2]
If credit rating is non-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	(200)
Fair value of fuel derivative level at which cash is pledged to CP	(600)
Counterparty B | Fuel derivatives
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]
Fair value of fuel derivatives - asset	15
Collateral Already Posted, Aggregate Fair Value	0
Letters of credit (LC)	0
Counterparty B | Minimum | 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	[1]
If credit rating is non-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		[3]
Fair value of fuel derivative level at which cash is received from CP		[3]
Counterparty C | Fuel derivatives
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]
Fair value of fuel derivatives - asset	33
Collateral Already Posted, Aggregate Fair Value	0
Letters of credit (LC)	0
Counterparty C | Minimum | 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	(75)	[4]
Option to substitute LC for cash Threshold 2	(550)	[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)	[1]
Fair value of fuel derivative levels at which cash is provided to CP Threshold 2	(550)	[1]
Fair value of fuel derivative level at which cash is received from CP	250	[1]
Fair value of fuel derivative levels at which cash is pledged to CP	(150)	[4]
If credit rating is non-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	0
Fair value of fuel derivative levels at which cash is provided to CP Threshold 2	(550)
Fair value of fuel derivative level at which cash is received from CP		[3]
Fair value of fuel derivative level at which cash is pledged to CP	(150)
Counterparty C | Maximum | 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	(150)	[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	(150)	[1]
Fair value of fuel derivative levels at which cash is pledged to CP	(550)	[4]
If credit rating is non-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	(150)
Fair value of fuel derivative level at which cash is pledged to CP	(550)
Counterparty D | Fuel derivatives
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]
Fair value of fuel derivatives - asset	14
Collateral Already Posted, Aggregate Fair Value	0
Letters of credit (LC)	0
Counterparty D | Minimum | 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	(125)	[5]
Option to substitute LC for cash Threshold 2	(550)	[5]
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)	[1]
Fair value of fuel derivative levels at which cash is provided to CP Threshold 2	(550)	[1]
Fair value of fuel derivative level at which cash is received from CP	(125)	[1]
Fair value of fuel derivative levels at which cash is pledged to CP	(150)	[4]
If credit rating is non-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	0
Fair value of fuel derivative levels at which cash is provided to CP Threshold 2	(550)
Fair value of fuel derivative level at which cash is received from CP		[3]
Fair value of fuel derivative level at which cash is pledged to CP	(150)
Counterparty D | Maximum | 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	(150)	[5]
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	(150)	[1]
Fair value of fuel derivative levels at which cash is pledged to CP	(550)	[4]
If credit rating is non-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	(150)
Fair value of fuel derivative level at which cash is pledged to CP	(550)
Counterparty E | Fuel derivatives
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]
Fair value of fuel derivatives - asset	17
Collateral Already Posted, Aggregate Fair Value	0
Letters of credit (LC)	0
Counterparty E | Minimum | 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		[5]
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	[1]
If credit rating is non-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		[3]
Fair value of fuel derivative level at which cash is received from CP		[3]
Counterparty F | Fuel derivatives
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]
Fair value of fuel derivatives - asset	11
Collateral Already Posted, Aggregate Fair Value	0
Letters of credit (LC)	0
Counterparty F | Minimum | 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)	[1]
Fair value of fuel derivative level at which cash is received from CP	70	[1]
If credit rating is non-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		[3]
Fair value of fuel derivative level at which cash is received from CP		[3]
Counterparty Other | Fuel derivatives
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]
Fair value of fuel derivatives - asset	11	[6]
Collateral Already Posted, Aggregate Fair Value	0	[6]
Letters of credit (LC)	$ 0	[6]

[1] Thresholds may vary based on changes in credit ratings within investment grade.

[2] The Company has the option of providing cash as collateral.

[3] Cash collateral is provided at 100 percent of fair value of fuel derivative contracts.

[4] The Company has the option of providing cash or letters of credit as collateral.

[5] The Company has the option to substitute letters of credit for 100 percent of cash collateral requirement.

[6] Individual counterparties with fair value of fuel derivatives <$10 million.