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Pension and Other Postretirement Plans - Participation in the IAM National Pension Plan (Details) - IAM National Pension Plan - Pension Benefits - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Multiemployer Plans [Line Items] |  |  |
Plan length | 10 years |  |
Additional contribution as a percentage of the hourly contribution rate | 0.025% |  |
Surcharge imposed | No |  |
United Airlines, Inc. |  |  |
Multiemployer Plans [Line Items] |  |  |
Contributions | $ 53 | $ 59 | $ 52

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