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Pension, Postretirement And Other Employee Benefit Plans - Narrative (Details) | 3 Months Ended | 12 Months Ended
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Mar. 31, 2021 USD ($) | Dec. 31, 2023 USD ($) plan | Dec. 31, 2022 USD ($) | Dec. 31, 2021 USD ($)
Schedule Of Defined Benefit Plans And Defined Contribution Plans Disclosures [Line Items] |  |  |  |
Number of defined benefit pension plans | plan |  | 2 |  |
Profit sharing and payroll tax expense |  | $ 681,000,000 | $ 133,000,000 | $ 0
IAM National Pension Plan |  |  |  |
Schedule Of Defined Benefit Plans And Defined Contribution Plans Disclosures [Line Items] |  |  |  |
Multi-employer plan contributions |  |  | 533,000,000 |
United Airlines, Inc. |  |  |  |
Schedule Of Defined Benefit Plans And Defined Contribution Plans Disclosures [Line Items] |  |  |  |
Expense for defined contribution plans |  | $ 960,000,000 | 756,000,000 | 651,000,000
Minimum |  |  |  |
Schedule Of Defined Benefit Plans And Defined Contribution Plans Disclosures [Line Items] |  |  |  |
Percentage of pre-tax earnings paid for profit sharing plan |  | 5.00% |  |
Minimum | United Airlines, Inc. |  |  |  |
Schedule Of Defined Benefit Plans And Defined Contribution Plans Disclosures [Line Items] |  |  |  |
Employer contribution percentage |  | 1.00% |  |
Maximum |  |  |  |
Schedule Of Defined Benefit Plans And Defined Contribution Plans Disclosures [Line Items] |  |  |  |
Percentage of pre-tax earnings paid for profit sharing plan |  | 20.00% |  |
Maximum | United Airlines, Inc. |  |  |  |
Schedule Of Defined Benefit Plans And Defined Contribution Plans Disclosures [Line Items] |  |  |  |
Employer contribution percentage |  | 16.00% |  |
Other Postretirement Benefits |  |  |  |
Schedule Of Defined Benefit Plans And Defined Contribution Plans Disclosures [Line Items] |  |  |  |
Special termination benefits |  | $ 0 | $ 0 | (31,000,000)
Defined benefit plan, expected future employer contributions, next fiscal year |  | $ 104,000,000 |  |
Employee Separation | Voluntary Separation Leave Programs |  |  |  |
Schedule Of Defined Benefit Plans And Defined Contribution Plans Disclosures [Line Items] |  |  |  |
Special termination benefits |  |  |  | 31,000,000
Employee Separation | Voluntary Separation Leave Programs | Full-Time Employees |  |  |  |
Schedule Of Defined Benefit Plans And Defined Contribution Plans Disclosures [Line Items] |  |  |  |
Restructuring programs, one-time contribution per employee | $ 125,000 |  |  | 125,000
Employee Separation | Voluntary Separation Leave Programs | Part-Time Employees |  |  |  |
Schedule Of Defined Benefit Plans And Defined Contribution Plans Disclosures [Line Items] |  |  |  |
Restructuring programs, one-time contribution per employee | $ 75,000 |  |  | $ 75,000

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