FAIR VALUE MEASUREMENTS - Fair Value Measurements on a Recurring Basis (Details) - Recurring - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Cash equivalents	$ 2,021	$ 5,450
Restricted cash equivalents	206	635
Long-term investments	1,450	1,459
U.S. Government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short-term investments	1,587	3,386
Corporate obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short-term investments	1,614
Fuel hedge contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Hedge derivatives, net	(47)	(18)
Other fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short-term investments	67
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Cash equivalents	2,021	5,450
Restricted cash equivalents	206	635
Long-term investments	1,305	1,326
Level 1 | U.S. Government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short-term investments	122	1,376
Level 1 | Corporate obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short-term investments	0
Level 1 | Fuel hedge contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Hedge derivatives, net	0	0
Level 1 | Other fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short-term investments	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Cash equivalents	0	0
Restricted cash equivalents	0	0
Long-term investments	38	36
Level 2 | U.S. Government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short-term investments	1,465	2,010
Level 2 | Corporate obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short-term investments	1,614
Level 2 | Fuel hedge contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Hedge derivatives, net	(47)	(18)
Level 2 | Other fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short-term investments	67
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Cash equivalents	0	0
Restricted cash equivalents	0	0
Long-term investments	107	97
Level 3 | U.S. Government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short-term investments	0	0
Level 3 | Corporate obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short-term investments	0
Level 3 | Fuel hedge contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Hedge derivatives, net	0	$ 0
Level 3 | Other fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short-term investments	$ 0