FAIR VALUE MEASUREMENTS - Measured at Fair Value on a Recurring Basis (Details) - Recurring - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Fair Value
Cash equivalents	$ 2,231	$ 2,021
Restricted cash equivalents	213	206
Long-term investments	1,572	1,450
Fuel hedge contracts	(6)	(47)
Short-term investments
Fair Value
Short-term investments	1,501	1,587
Corporate obligations
Fair Value
Short-term investments	1,756	1,614
Asset-backed securities
Fair Value
Short-term investments	37
Other fixed income securities
Fair Value
Short-term investments	102	67
Level 1
Fair Value
Cash equivalents	2,231	2,021
Restricted cash equivalents	213	206
Long-term investments	1,424	1,305
Fuel hedge contracts	0	0
Level 1 | Short-term investments
Fair Value
Short-term investments	162	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	38	38
Fuel hedge contracts	(6)	(47)
Level 2 | Short-term investments
Fair Value
Short-term investments	1,339	1,465
Level 2 | Corporate obligations
Fair Value
Short-term investments	1,756	1,614
Level 2 | Asset-backed securities
Fair Value
Short-term investments	37
Level 2 | Other fixed income securities
Fair Value
Short-term investments	102	67
Level 3
Fair Value
Cash equivalents	0	0
Restricted cash equivalents	0	0
Long-term investments	110	107
Fuel hedge contracts	0	0
Level 3 | Short-term investments
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