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Commitments and Contingencies - Aircraft Purchase Commitments By Period (Details)
$ in Millions
Mar. 31, 2019
USD ($)
Unrecorded Unconditional Purchase Obligation  
Percentage of mainline fleet to be replaced by 2023 (percent) 25.00%
Future aircraft purchase commitments  
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity  
Nine months ending December 31, 2019 $ 2,110
2020 3,140
2021 3,260
2022 2,790
2023 1,850
Thereafter 1,940
Total $ 15,090