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Contractual Principal Payments (Detail) - UAL and United - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Debt Instrument [Line Items]    
2018 $ 1,565  
2019 1,165  
2020 1,170  
2021 1,157  
2022 1,492  
After 2022 6,882  
Long-term debt $ 13,431 $ 10,891