Operating Leases - Aircraft Rent Expense and Maintenance Obligations (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024	Dec. 31, 2024
Lessee, Lease, Description [Line Items]
Aircraft rent expense	$ 194	$ 147	$ 355	$ 306
Supplemental rent expense related to probable lease return condition obligations	15	1	3	14
Leased aircraft return cost liability	31		31		$ 49
Aircraft maintenance deposit refunds				104
Aircraft
Lessee, Lease, Description [Line Items]
Aircraft rent expense (benefit)	$ 0	$ 11	$ 20	$ 14