ACCUMULATED OTHER COMPREHENSIVE INCOME - Reclassifications (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Fuel and oil	$ 6,217	$ 5,975	$ 3,310
Other (gains) losses, net	(62)	12	(50)
Less: Tax expense	168	189	348
Net income (loss)	465	539	977
Other operating expenses	3,999	$ 3,735	$ 2,394
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income (loss)	(104)
Defined benefit plan items | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other (gains) losses, net	(10)
Less: Tax expense	(3)
Net income (loss)	(7)
Interest rate derivatives | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other (gains) losses, net	0
Less: Tax expense	4
Net income (loss)	(4)
Fuel derivative contracts | Fuel and Interest Rate Derivatives | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Fuel and oil	(128)
Less: Tax expense	(29)
Net income (loss)	(99)
Interest rate derivatives | Fuel and Interest Rate Derivatives | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Less: Tax expense	1
Net income (loss)	6
Other operating expenses	$ 7