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Debt - Contractual Principal Payments under Outstanding Long-Term Debt Agreements (Details) - UAL and United - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Debt Instrument [Line Items]    
2019 $ 1,230  
2020 1,310  
2021 1,300  
2022 1,653  
2023 703  
After 2023 7,440  
Long-term debt $ 13,636 $ 13,431