Financial Derivative Instruments - Fair Values of Fuel Derivatives Amounts Posted as Collateral (Details) $ in Millions	Sep. 30, 2020 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	$ 109
Cash collateral held (from) by CP	27
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]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	24
Cash collateral held (from) by CP	27
Letters of credit (LC)	0
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	14
Cash collateral held (from) by CP	0
Letters of credit (LC)	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	28
Cash collateral held (from) by CP	0
Letters of credit (LC)	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	12
Cash collateral held (from) by CP	0
Letters of credit (LC)	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	11
Cash collateral held (from) by CP	0
Letters of credit (LC)	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	10
Cash collateral held (from) by CP	0
Letters of credit (LC)	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	10	[1]
Cash collateral held (from) by CP	0	[1]
Letters of credit (LC)	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]
Fair value of fuel derivative levels at which cash collateral is pledged to CP	(200)	[3]
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		[4]
Fair value of fuel derivative levels at which cash collateral is pledged to CP	(200)
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 levels at which cash is provided to CP Threshold 1		[4]
Fair value of fuel derivative level at which cash is received from CP		[4]
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	(75)	[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	(75)	[2]
Fair value of fuel derivative levels at which cash is provided to CP Threshold 2	(550)	[2]
Fair value of fuel derivative level at which cash is received from CP	250	[2]
Fair value of fuel derivative levels at which cash collateral is pledged to CP	(150)	[5]
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		[4]
Fair value of fuel derivative levels at which cash collateral is pledged to CP	(150)
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	(125)	[6]
Option to substitute LC for cash Threshold 2	(550)	[6]
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)	[2]
Fair value of fuel derivative levels at which cash is provided to CP Threshold 2	(550)	[2]
Fair value of fuel derivative level at which cash is received from CP	(125)	[2]
Fair value of fuel derivative levels at which cash collateral is pledged to CP	(150)	[5]
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		[4]
Fair value of fuel derivative levels at which cash collateral is pledged to CP	(150)
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		[6]
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 levels at which cash is provided to CP Threshold 1		[4]
Fair value of fuel derivative level at which cash is received from CP		[4]
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)	[2]
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 levels at which cash is provided to CP Threshold 1		[4]
Fair value of fuel derivative level at which cash is received from CP		[4]
Maximum | 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 collateral is pledged to CP	(600)	[3]
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 levels at which cash collateral is pledged to CP	(600)
Maximum | 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	(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)	[2]
Fair value of fuel derivative levels at which cash collateral is pledged to CP	(550)	[5]
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 levels at which cash collateral is pledged to CP	(550)
Maximum | 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	(150)	[6]
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)	[2]
Fair value of fuel derivative levels at which cash collateral is pledged to CP	(550)	[5]
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 levels at which cash collateral is pledged to CP	$ (550)

[1] Individual counterparties with fair value of fuel derivatives < $9 million.

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

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

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

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

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