Financial Derivative Instruments - Fair Values of Fuel Derivatives Amounts Posted as Collateral (Details) - Fuel derivatives $ in Millions	Mar. 31, 2018 USD ($)
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	$ 340
Cash collateral held from (by) CP	80
Aircraft collateral pledged to CP	0
Letters of credit (LC)	0
Counterparty A
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	116
Cash collateral held from (by) CP	80
Aircraft collateral pledged to CP	0
Letters of credit (LC)	0
Counterparty B
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	67
Cash collateral held from (by) CP	0
Aircraft collateral pledged to CP	0
Letters of credit (LC)	0
Counterparty C
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	77
Cash collateral held from (by) CP	0
Aircraft collateral pledged to CP	0
Letters of credit (LC)	0
Counterparty D
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	39
Cash collateral held from (by) CP	0
Aircraft collateral pledged to CP	0
Letters of credit (LC)	0
Counterparty E
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	17
Cash collateral held from (by) CP	0
Aircraft collateral pledged to CP	0
Letters of credit (LC)	0
Counterparty F
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
Aircraft collateral pledged to CP	0
Letters of credit (LC)	0
Counterparty Other
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	13	[1]
Cash collateral held from (by) CP	0	[1]
Aircraft collateral pledged to CP	0	[1]
Letters of credit (LC)	0	[1]
Minimum | Counterparty A
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]
Option to substitute LC for aircraft	(200)	[2]
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 levels at which cash is provided to CP Threshold 2	(600)
Fair value of fuel derivative level at which cash is received from CP	50	[3]
Fair value of fuel derivative levels at which aircraft or cash collateral is pledged to CP	(200)	[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	(600)
Fair value of fuel derivative level at which cash is received from CP		[5]
Fair value of fuel derivative levels at which aircraft or cash collateral is pledged to CP	(200)
Minimum | Counterparty B
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]
Option to substitute LC for aircraft	(100)	[6]
Fair Value Of Fuel Derivative Levels In Which Letters Of Credit May Be Substituted For Cash Threshold 1	(500)	[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	(50)
Fair value of fuel derivative levels at which cash is provided to CP Threshold 2	(500)
Fair value of fuel derivative level at which cash is received from CP	150	[3]
Fair value of fuel derivative levels at which aircraft or cash collateral is pledged to CP	(100)	[6]
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	(500)
Fair value of fuel derivative level at which cash is received from CP		[5]
Fair value of fuel derivative levels at which aircraft or cash collateral is pledged to CP	(100)
Minimum | Counterparty C
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]
Option to substitute LC for aircraft	(150)	[6]
Fair Value Of Fuel Derivative Levels In Which Letters Of Credit May Be Substituted For Cash Threshold 1	(75)	[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	(75)	[3]
Fair value of fuel derivative levels at which cash is provided to CP Threshold 2	(550)	[3]
Fair value of fuel derivative level at which cash is received from CP	250	[3]
Fair value of fuel derivative levels at which aircraft or cash collateral is pledged to CP	(150)	[6]
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		[5]
Fair value of fuel derivative levels at which aircraft or cash collateral is pledged to CP	(150)
Minimum | Counterparty D
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]
Option to substitute LC for aircraft	(150)	[6]
Fair Value Of Fuel Derivative Levels In Which Letters Of Credit May Be Substituted For Cash Threshold 1	(125)	[7]
Option to substitute LC for cash Threshold 2	(550)	[7]
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)	[3]
Fair value of fuel derivative levels at which cash is provided to CP Threshold 2	(500)
Fair value of fuel derivative level at which cash is received from CP	75	[3]
Fair value of fuel derivative levels at which aircraft or cash collateral is pledged to CP	(150)	[6]
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		[5]
Fair value of fuel derivative levels at which aircraft or cash collateral is pledged to CP	(150)
Minimum | Counterparty E
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]
Fair Value Of Fuel Derivative Levels In Which Letters Of Credit May Be Substituted For Cash Threshold 1		[7]
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	(100)	[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		[5]
Fair value of fuel derivative level at which cash is received from CP		[5]
Minimum | Counterparty F
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)	[3]
Fair value of fuel derivative level at which cash is received from CP	(30)	[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		[5]
Fair value of fuel derivative level at which cash is received from CP		[5]
Maximum | Counterparty A
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]
Option to substitute LC for aircraft	(600)	[2]
Fair Value Of Fuel Derivative Levels In Which Letters Of Credit May Be Substituted For Cash Threshold 1
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	(200)
Fair value of fuel derivative levels at which aircraft or cash collateral is pledged to CP	(600)	[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	(200)
Fair value of fuel derivative levels at which aircraft or cash collateral is pledged to CP	(600)
Maximum | Counterparty B
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]
Option to substitute LC for aircraft	(500)	[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	(100)
Fair value of fuel derivative levels at which aircraft or cash collateral is pledged to CP	(500)	[6]
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	(100)
Fair value of fuel derivative levels at which aircraft or cash collateral is pledged to CP	(500)
Maximum | Counterparty C
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]
Option to substitute LC for aircraft	(550)	[6]
Fair Value Of Fuel Derivative Levels In Which Letters Of Credit May Be Substituted 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)
Fair value of fuel derivative levels at which aircraft or cash collateral is pledged to CP	(550)	[6]
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 aircraft or cash collateral is pledged to CP	(550)
Maximum | Counterparty D
Schedule Of Fair Values Of Fuel Derivatives Amounts Posted As Collateral And Collateral Posting Thresholds [Line Items]
Option to substitute LC for aircraft	(550)	[6]
Fair Value Of Fuel Derivative Levels In Which Letters Of Credit May Be Substituted For Cash Threshold 1	(150)	[7]
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)
Fair value of fuel derivative levels at which aircraft or cash collateral is pledged to CP	(550)	[6]
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 aircraft or cash collateral is pledged to CP	$ (550)

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

[2] The Company has the option of providing letters of credit in addition to aircraft collateral if the appraised value of the aircraft does not meet the collateral requirements.

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

[4] The Company has the option of providing cash or pledging aircraft as collateral.

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

[6] The Company has the option of providing cash, letters of credit, or pledging aircraft as collateral.

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