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Debt - Contractual Principal Payments (Details) - UAL And United
$ in Millions
Mar. 31, 2020
USD ($)
Debt Instrument [Line Items]  
Last nine months of 2020 $ 1,181
2021 3,948
2022 1,799
2023 849
2024 3,155
After 2024 6,524
Long-term debt $ 17,456