OPERATING SEGMENT INFORMATION (Tables) - USD ($)
$ in Millions

3 Months Ended

6 Months Ended

Jun. 30, 2024

Jun. 30, 2023

Jun. 30, 2024

Jun. 30, 2023

Segment Reporting [Abstract]

Schedule of Segment Reporting Information, by Segment

Operating segment information is as follows (in millions):

	Three Months Ended June 30, 2024
	Mainline		Regional		Horizon		Consolidating & Other(a)		Air Group Adjusted(b)			Special Items(c)		Consolidated
Operating Revenue
Passenger revenue	$	2,188	$	463	$	—	$	—	$	2,651		$	—	$	2,651
CPA revenue	—		—		112		(112)		—			—		—
Mileage Plan other revenue	160		14		—		—		174			—		174
Cargo and other revenue	69		—		—		3		72			—		72
Total Operating Revenue	2,417		477		112		(109)		2,897			—		2,897
Operating Expenses
Operating expenses, excluding fuel	1,516		322		92		(116)		1,814			146		1,960
Fuel expense	520		100		—		—		620			(5)		615
Total Operating Expenses	2,036		422		92		(116)		2,434			141		2,575
Non-operating Income (Expense)	6		—		(12)		—		(6)			—		(6)
Income (Loss) Before Income Tax	$	387	$	55	$	8	$	7	$	457		$	(141)	$	316
Pretax Margin									15.8		%			10.9		%
	Three Months Ended June 30, 2023
	Mainline		Regional		Horizon		Consolidating & Other(a)		Air Group Adjusted(b)			Special Items(c)		Consolidated
Operating Revenue
Passenger revenue	$	2,209	$	389	$	—	$	—	$	2,598		$	—	$	2,598
CPA revenue	—		—		92		(92)		—			—		—
Mileage Plan other revenue	158		12		—		—		170			—		170
Cargo and other revenue	67		—		—		3		70			—		70
Total Operating Revenue	2,434		401		92		(89)		2,838			—		2,838
Operating Expenses
Operating expenses, excluding fuel	1,468		279		87		(92)		1,742			186		1,928
Fuel expense	490		81		—		1		572			1		573
Total Operating Expenses	1,958		360		87		(91)		2,314			187		2,501
Non-operating Income (Expense)	3		—		(10)		1		(6)			(6)		(12)
Income (Loss) Before Income Tax	$	479	$	41	$	(5)	$	3	$	518		$	(193)	$	325
Pretax Margin									18.3		%			11.5		%

	Six Months Ended June 30, 2024
	Mainline		Regional		Horizon		Consolidating & Other(a)		Air Group Adjusted(b)			Special Items(c)		Consolidated
Operating Revenue
Passenger revenue	$	3,817	$	838	$	—	$	—	$	4,655		$	—	$	4,655
CPA revenue	—		—		216		(216)		—			—		—
Mileage Plan other revenue	309		29		—		—		338			—		338
Cargo and other revenue	131		—		—		5		136			—		136
Total Operating Revenue	4,257		867		216		(211)		5,129			—		5,129
Operating Expenses
Operating expenses, excluding fuel	3,030		621		180		(218)		3,613			180		3,793
Fuel expense	1,005		193		—		—		1,198			(18)		1,180
Total Operating Expenses	4,035		814		180		(218)		4,811			162		4,973
Non-operating Income (Expense)	3		—		(23)		2		(18)			—		(18)
Income (Loss) Before Income Tax	$	225	$	53	$	13	$	9	$	300		$	(162)	$	138
Pretax Margin									5.8		%			2.7		%

	Six Months Ended June 30, 2023
	Mainline		Regional		Horizon		Consolidating & Other(a)		Air Group Adjusted(b)			Special Items(c)		Consolidated
Operating Revenue
Passenger revenue	$	3,882	$	700	$	—	$	—	$	4,582		$	—	$	4,582
CPA revenue	—		—		170		(170)		—			—		—
Mileage Plan other revenue	301		23		—		—		324			—		324
Cargo and other revenue	124		—		—		4		128			—		128
Total Operating Revenue	4,307		723		170		(166)		5,034			—		5,034
Operating Expenses
Operating expenses, excluding fuel	2,858		535		171		(169)		3,395			250		3,645
Fuel expense	1,051		166		—		—		1,217			21		1,238
Total Operating Expenses	3,909		701		171		(169)		4,612			271		4,883
Non-operating Income (Expense)	(3)		—		(18)		2		(19)			(6)		(25)
Income (Loss) Before Income Tax	$	395	$	22	$	(19)	$	5	$	403		$	(277)	$	126
Pretax Margin									8.0		%			2.5		%

Nonoperating Income (Expense)

$ (6)

$ (12)

$ (18)

$ (25)

Interest income

24

22

41

39

Operating Expenses, Excluding Fuel

1,960

1,928

3,793

3,645

Economic fuel

615

573

1,180

1,238

Operating Expenses

2,575

2,501

4,973

4,883

Income (Loss) Before Income Tax

$ 316

$ 325

$ 138

$ 126

Pretax Margin, GAAP

10.90%

11.50%

2.70%

2.50%

Pretax Margin, Adjusted

15.80%

18.30%

5.80%

8.00%