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Debt - Narrative (Details) - USD ($) |  |  |  | 2 Months Ended | 3 Months Ended | 12 Months Ended |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Sep. 28, 2020 | Jul. 02, 2020 | Apr. 20, 2020 | Feb. 25, 2021 | Dec. 31, 2020 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Feb. 15, 2021 | Jan. 15, 2021 | Jun. 30, 2020 | Apr. 07, 2020 | Mar. 20, 2020 | Mar. 09, 2020
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total funding provided under the Payroll Support Program |  |  | $ 5,100,000,000 |  |  |  |  |  |  |  |  |  |  |
Funding provided through loans under the Payroll Support Program |  |  | $ 1,500,000,000 |  |  |  |  |  |  |  |  |  |  |
Proceeds from issuance of debt |  |  |  |  |  | $ 16,044,000,000 | $ 1,847,000,000 | $ 1,594,000,000 |  |  |  |  |  |
Long term debt |  |  |  |  | $ 27,301,000,000 | 27,301,000,000 | 14,733,000,000 |  |  |  |  |  |  |
Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total funding provided under the Payroll Support Program |  |  |  |  |  |  |  |  |  | $ 2,600,000,000 |  |  |  |
United Airlines, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from issuance of debt |  |  |  |  |  | $ 16,044,000,000 | 1,847,000,000 | $ 1,594,000,000 |  |  |  |  |  |
Secured Debt | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fixed interest rate |  |  |  |  | 1.49% | 1.49% |  |  |  |  |  |  |  |
Long term debt |  |  |  |  | $ 1,000,000,000 | $ 1,000,000,000 | 0 |  |  |  |  |  |  |
Secured Debt | Revolving Credit Facility | United Airlines, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Credit agreement |  |  |  |  | 2,000,000,000.0 | 2,000,000,000.0 |  |  |  |  |  |  |  |
Available under revolving credit facility |  |  |  |  | $ 1,000,000,000.0 | 1,000,000,000.0 |  |  |  |  |  |  |  |
Proceeds from lines of credit |  | $ 1,000,000,000.0 |  |  |  | $ 1,000,000,000.0 |  |  |  |  |  |  |  |
Secured Debt | LIBOR | Revolving Credit Facility | United Airlines, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis spread on variable rate |  | 2.25% |  |  |  |  |  |  |  |  |  |  |  |
Variable rate floor |  | 0.00% |  |  |  |  |  |  |  |  |  |  |  |
Secured Debt | Alternative Rate Based on Certain Market Interest Rates | Revolving Credit Facility | United Airlines, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis spread on variable rate |  | 1.25% |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commitment fee percentage |  | 0.75% |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit | Revolving Credit Facility | United Airlines, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Available under revolving credit facility |  | $ 1,000,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |
PSP Note | Unsecured Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Term of debt |  |  | 10 years |  |  |  |  |  |  |  |  |  |  |
Fixed interest rate |  |  |  |  | 1.00% | 1.00% |  |  |  |  |  |  |  |
Long term debt |  |  |  |  | $ 1,501,000,000 | $ 1,501,000,000 | 0 |  |  |  |  |  |  |
PSP Note | Unsecured Debt | Years One through Five |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fixed interest rate |  |  |  |  | 1.00% | 1.00% |  |  |  |  |  |  |  |
PSP Note | Unsecured Debt | Years Six through Ten | SOFR |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis spread on variable rate |  |  |  |  | 2.00% |  |  |  |  |  |  |  |  |
Mileage Plus Senior Secured Notes | Secured Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  | $ 3,800,000,000 |  |  |  |  |  |  |  |  |  |  |  |
Fixed interest rate |  | 6.50% |  |  | 6.50% | 6.50% |  |  |  |  |  |  |  |
Long term debt |  |  |  |  | $ 3,800,000,000 | $ 3,800,000,000 | 0 |  |  |  |  |  |  |
MileagePlus Term Loan Facility | Secured Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fixed interest rate |  |  |  |  | 5.49% | 5.49% |  |  |  |  |  |  |  |
Long term debt |  |  |  |  | $ 3,000,000,000 | $ 3,000,000,000 | 0 |  |  |  |  |  |  |
MileagePlus Term Loan Facility | Line of Credit |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  | $ 3,000,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from issuance of debt |  | $ 3,000,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |
MileagePlus Term Loan Facility | Line of Credit | LIBOR |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis spread on variable rate |  | 5.25% |  |  |  |  |  |  |  |  |  |  |  |
Variable rate floor |  | 1.00% |  |  |  |  |  |  |  |  |  |  |  |
CARES Act Credit Agreement | Line of Credit |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount | $ 520,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from issuance of debt | 520,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Credit agreement | $ 7,500,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |
CARES Act Credit Agreement | Line of Credit | LIBOR |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis spread on variable rate | 3.00% |  |  |  |  |  |  |  |  |  |  |  |  |
Variable rate floor | 0.00% |  |  |  |  |  |  |  |  |  |  |  |  |
Used Aircraft Facility | Secured Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,000,000,000
Long term debt |  |  |  |  | 14,538,000,000 | 14,538,000,000 | $ 11,585,000,000 |  |  |  |  |  |  |
Spare Parts Facility | Secured Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  |  | $ 500,000,000 |
Spare Engines Facility | Secured Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  | $ 250,000,000 |  |
Term Loan Credit and Guaranty Agreement | Bridge Loan |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  | $ 200,000,000 |  |  |
Pass-Through Certificates |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long term debt |  |  |  |  | 12,100,000,000 | 12,100,000,000 |  |  |  |  |  |  |  |
Borrowings Financing Aircraft Purchases | Secured Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  | 691,000,000 | 691,000,000 |  |  |  |  |  |  |  |
Special Facility Revenue Bonds | Secured Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  | $ 295,000,000 | $ 295,000,000 |  |  |  |  |  |  |  |
Fixed interest rate |  |  |  |  | 4.00% | 4.00% |  |  |  |  |  |  |  |
Special Facility Revenue Bonds | Mortgage |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long term debt |  |  |  |  | $ 159,000,000 | $ 159,000,000 |  |  |  |  |  |  |  |
PSP2 Note | Unsecured Debt | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Funding provided through loans under the Payroll Support Program |  |  |  |  |  |  |  |  | $ 753,000,000 |  |  |  |  |
Term of debt |  |  |  | 10 years |  |  |  |  |  |  |  |  |  |
Increase in principal amount as a percent of treasury disbursements |  |  |  |  |  |  |  |  | 30.00% |  |  |  |  |
PSP2 Note | Unsecured Debt | Years One through Five | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fixed interest rate |  |  |  |  |  |  |  |  | 1.00% |  |  |  |  |
PSP2 Note | Unsecured Debt | Years Six through Ten | SOFR | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis spread on variable rate |  |  |  | 2.00% |  |  |  |  |  |  |  |  |  |

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