LEASES - Related Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Assets
Accumulated amortization	$ 477	$ 634
Operating	1,223	1,394
Finance	125	297
Total lease assets	$ 1,348	$ 1,691
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration]	Flight equipment	Flight equipment
Current
Operating	$ 208	$ 225
Finance	$ 29	$ 42
Finance Lease, Liability, Current, Statement of Financial Position [Extensible Enumeration]	Long-Term Debt and Lease Obligation, Current	Long-Term Debt and Lease Obligation, Current
Noncurrent
Operating	$ 985	$ 1,118
Finance	$ 75	$ 147
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible Enumeration]	Long-term debt less current maturities	Long-term debt less current maturities
Total lease liabilities	$ 1,297	$ 1,532