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Commitments and Contingencies - Aircraft Purchase Commitments By Period (Details) - Future aircraft purchase commitments
$ in Millions
Sep. 30, 2017
USD ($)
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity  
Three months ending December 31, 2017 $ 920
2018 3,480
2019 3,090
2020 2,160
2021 2,090
Thereafter 1,330
Total $ 13,070