FAIR VALUE MEASUREMENTS - Measured at Fair Value on a Recurring Basis (Details) - Recurring - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Fair Value
Cash equivalents	$ 2,495	$ 1,545
Restricted cash equivalents	501	653
Long-term investments and related	2,641	2,867
Fuel hedge contracts	(22)	5
U.S. Government securities
Fair Value
Short-term investments	473	859
Corporate obligations
Fair Value
Short-term investments	86	218
Other fixed income securities
Fair Value
Short-term investments	30	50
Level 1
Fair Value
Cash equivalents	2,495	1,545
Restricted cash equivalents	501	653
Long-term investments and related	2,387	2,614
Fuel hedge contracts	0	0
Level 1 | U.S. Government securities
Fair Value
Short-term investments	101	204
Level 1 | Corporate obligations
Fair Value
Short-term investments	0	0
Level 1 | Other fixed income securities
Fair Value
Short-term investments	0	0
Level 2
Fair Value
Cash equivalents	0	0
Restricted cash equivalents	0	0
Long-term investments and related	139	134
Fuel hedge contracts	(22)	5
Level 2 | U.S. Government securities
Fair Value
Short-term investments	372	655
Level 2 | Corporate obligations
Fair Value
Short-term investments	86	218
Level 2 | Other fixed income securities
Fair Value
Short-term investments	30	50
Level 3
Fair Value
Cash equivalents	0	0
Restricted cash equivalents	0	0
Long-term investments and related	115	119
Fuel hedge contracts	0	0
Level 3 | U.S. Government securities
Fair Value
Short-term investments	0	0
Level 3 | Corporate obligations
Fair Value
Short-term investments	0	0
Level 3 | Other fixed income securities
Fair Value
Short-term investments	$ 0	$ 0