Leases and Capacity Purchase Agreements - Narrative (Details) $ in Millions	1 Months Ended	12 Months Ended
	Jan. 31, 2024 USD ($)	Dec. 31, 2023 USD ($) aircraft lease	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Lessee, Lease, Description [Line Items]
Number of aircraft leases under various sale-leaseback transactions | lease		42
Additional leases that have not yet commenced		$ 1,600
Lease terms of additional leases that have not yet commenced (up to)		12 years
Notes receivable		$ 103
Liabilities due		3,835	$ 3,395
Capacity Purchase Agreements | Subsequent Event
Lessee, Lease, Description [Line Items]
Change in CPA future commitments	$ 600
United Airlines, Inc.
Lessee, Lease, Description [Line Items]
Liabilities due		$ 3,835	3,395
Cash Collateralized Letters of Credit | United Airlines, Inc.
Lessee, Lease, Description [Line Items]
Number of regional aircraft | aircraft		413
Mainline Aircraft
Lessee, Lease, Description [Line Items]
Number of aircraft subject to operating leases | aircraft		70
Number of aircraft subject to finance leases | aircraft		22
Mainline Aircraft | Minimum
Lessee, Lease, Description [Line Items]
Initial term of operating leases		1 month
Mainline Aircraft | Maximum
Lessee, Lease, Description [Line Items]
Initial term of operating leases		12 years
Regional Aircraft
Lessee, Lease, Description [Line Items]
Number of aircraft subject to operating leases | aircraft		275
Number of aircraft subject to finance leases | aircraft		13
Regional Aircraft | United Airlines, Inc. | Capacity Purchase Agreements
Lessee, Lease, Description [Line Items]
Expenses		$ 1,100	900	$ 600
Regional Aircraft | United Airlines, Inc. | Capacity Purchase Agreements | Related Party
Lessee, Lease, Description [Line Items]
Notes receivable		84	62
Liabilities due		$ 122	$ 118
Non-Aircraft | Minimum
Lessee, Lease, Description [Line Items]
Initial term of operating leases		1 month
Non-Aircraft | Maximum
Lessee, Lease, Description [Line Items]
Initial term of operating leases		29 years