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Debt - Contractual Principal Payments (Details) - UAL And United
$ in Millions
Mar. 31, 2019
USD ($)
Debt Instrument [Line Items]  
Last nine months of 2019 $ 979
2020 1,344
2021 1,338
2022 1,690
2023 740
After 2023 8,033
Long-term debt $ 14,124