Pension, Postretirement And Other Employee Benefit Plans - Participation in the IAM National Pension Plan (Details) - IAM National Pension Plan - Pension Benefits - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Jan. 01, 2021
Multiemployer Plans [Line Items]
Plan length	10 years
Additional contribution as a percentage of the hourly contribution rate	2.50%
Surcharge imposed	No
United Airlines, Inc.
Multiemployer Plans [Line Items]
Funded percentage				87.10%
Contributions	$ 87	$ 75	$ 58