Leases and Capacity Purchase Agreements - Components of Lease Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Leases [Abstract]
Operating lease cost	$ 925	$ 941	$ 958
Variable and short-term lease cost	3,028	2,603	2,291
Amortization of finance lease assets	52	72	89
Interest on finance lease liabilities	20	13	16
Sublease income	(39)	(33)	(26)
Total lease cost	$ 3,986	$ 3,596	$ 3,328