Commitments and Contingencies - Aircraft Purchase Commitments By Aircraft (Details) - Future aircraft purchase commitments - aircraft	Sep. 30, 2017	Jun. 30, 2017
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	265
B-737-900ER
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	46
A321-200
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	96
A330-900neo
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	25
A350-900
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	23	25
CS100
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	75