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Debt - Contractual Principal Payments under Outstanding Long-Term Debt Agreements (Details) - UAL and United
$ in Millions
Dec. 31, 2019
USD ($)
Debt Instrument [Line Items]  
2020 $ 1,407
2021 1,415
2022 1,765
2023 815
2024 3,122
After 2024 6,209
Long-term debt $ 14,733