COMMITMENTS AND CONTINGENCIES - Narrative (Details) $ in Millions		3 Months Ended	12 Months Ended
	Dec. 31, 2023 USD ($)	Dec. 31, 2023 USD ($) aircraft	Dec. 31, 2023 USD ($) aircraft	Apr. 30, 2023 gate	Dec. 31, 2022 USD ($)	Mar. 31, 2022 gate
Other Commitments [Line Items]
Assets	$ 36,487	$ 36,487	$ 36,487		$ 35,369
Payment Guarantee
Other Commitments [Line Items]
Principal amount of guarantee	59	59	59
Net present value of future principal and interest payments	65	65	65		$ 77
Los Angeles World Airports
Other Commitments [Line Items]
New airport gates | gate				9
Payments for airport projects	62
Projected airport improvement costs	2,300	2,300	2,300
William P. Hobby Airport
Other Commitments [Line Items]
New airport gates | gate						7
Projected airport improvement costs	470	470	470
Assets	0	0	0
Aircraft
Other Commitments [Line Items]
Committed expenditures, year one	2,200	2,200	2,200
Committed expenditures, year two	1,700	1,700	1,700
Committed expenditures, year three	1,900	1,900	1,900
Committed expenditures, year four	2,600	2,600	2,600
Committed expenditures, year five	2,900	2,900	2,900
Committed expenditures, year six and beyond	$ 5,300	$ 5,300	$ 5,300
737 MAX 8
Other Commitments [Line Items]
Number of aircraft orders | aircraft			86
Number of aircraft orders accelerated | aircraft		1
Number of aircraft orders converted and shifted | aircraft		3
Number of aircraft orders shifted | aircraft		3
737 MAX 7
Other Commitments [Line Items]
Number of aircraft orders | aircraft			39
Number of aircraft orders exercised | aircraft		8
Number of aircraft orders converted and shifted | aircraft		3