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Income Taxes - Narrative (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Income Taxes [Line Items] |  |  |
Valuation allowance | $ 197 | $ (4) | $ (3)
Unrecognized tax benefits | 57 | 53 | 39
Unrecognized tax benefits that would impact effective tax rate if recognized | 57 |  |
Capital loss carryforwards, valuation allowance | 185 |  |
United Airlines, Inc. |  |  |
Income Taxes [Line Items] |  |  |
Valuation allowance | 197 | $ (4) | $ (3)
Federal |  |  |
Income Taxes [Line Items] |  |  |
Tax credits | 42 |  |
Federal | United Airlines, Inc. |  |  |
Income Taxes [Line Items] |  |  |
NOL carryforwards | 11,000 |  |
Tax effected amount of NOLs | 2,300 |  |
NOLs without an expiration date | $ 9,400 |  |
Federal | Minimum |  |  |
Income Taxes [Line Items] |  |  |
Tax credit expiration period | 1 year |  |
Federal | Maximum |  |  |
Income Taxes [Line Items] |  |  |
Tax credit expiration period | 18 years |  |
Federal | 2026 | United Airlines, Inc. |  |  |
Income Taxes [Line Items] |  |  |
NOL carryforwards | $ 100 |  |
Federal | 2028 | United Airlines, Inc. |  |  |
Income Taxes [Line Items] |  |  |
NOL carryforwards | 500 |  |
Federal | 2029 | United Airlines, Inc. |  |  |
Income Taxes [Line Items] |  |  |
NOL carryforwards | 400 |  |
Federal | 2032 | United Airlines, Inc. |  |  |
Income Taxes [Line Items] |  |  |
NOL carryforwards | 200 |  |
Federal | 2033 | United Airlines, Inc. |  |  |
Income Taxes [Line Items] |  |  |
NOL carryforwards | 400 |  |
State |  |  |
Income Taxes [Line Items] |  |  |
Tax credits | 33 |  |
NOL and tax credit carryforwards, valuation allowance | 62 |  |
State | United Airlines, Inc. |  |  |
Income Taxes [Line Items] |  |  |
NOL carryforwards | 3,700 |  |
Tax effected amount of NOLs | $ 200 |  |
State | Minimum |  |  |
Income Taxes [Line Items] |  |  |
Tax credit expiration period | 1 year |  |
State | Minimum | United Airlines, Inc. |  |  |
Income Taxes [Line Items] |  |  |
NOL expiration period | 5 years |  |
State | Maximum |  |  |
Income Taxes [Line Items] |  |  |
Tax credit expiration period | 11 years |  |
State | Maximum | United Airlines, Inc. |  |  |
Income Taxes [Line Items] |  |  |
NOL expiration period | 20 years |  |

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