FAIR VALUE MEASUREMENTS - Measured at Fair Value on a Recurring Basis (Details) - Recurring - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Fair Value
Cash equivalents	$ 1,339	$ 2,021
Restricted cash equivalents	156	206
Long-term investments	1,701	1,450
Fuel hedge contracts	(9)	(47)
U.S. Government securities
Fair Value
Short-term investments	1,498	1,587
Corporate obligations
Fair Value
Short-term investments	1,643	1,614
Asset-backed securities
Fair Value
Short-term investments	120
Other fixed income securities
Fair Value
Short-term investments	107	67
Level 1
Fair Value
Cash equivalents	1,339	2,021
Restricted cash equivalents	156	206
Long-term investments	1,536	1,305
Fuel hedge contracts	0	0
Level 1 | U.S. Government securities
Fair Value
Short-term investments	205	122
Level 1 | Corporate obligations
Fair Value
Short-term investments	0	0
Level 1 | Asset-backed securities
Fair Value
Short-term investments	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	39	38
Fuel hedge contracts	(9)	(47)
Level 2 | U.S. Government securities
Fair Value
Short-term investments	1,293	1,465
Level 2 | Corporate obligations
Fair Value
Short-term investments	1,643	1,614
Level 2 | Asset-backed securities
Fair Value
Short-term investments	120
Level 2 | Other fixed income securities
Fair Value
Short-term investments	107	67
Level 3
Fair Value
Cash equivalents	0	0
Restricted cash equivalents	0	0
Long-term investments	126	107
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 | Asset-backed securities
Fair Value
Short-term investments	0
Level 3 | Other fixed income securities
Fair Value
Short-term investments	$ 0	$ 0