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Commitments and Contingencies - Future aircraft purchase commitments (Details) - Capital Addition Purchase Commitments
$ in Millions
Mar. 31, 2018
USD ($)
Unrecorded Unconditional Purchase Obligation [Line Items]  
Remainder of 2018 $ 866
2019 900
2020 1,386
2021 1,681
2022 1,212
After 2022 $ 5,091