Commitments and Contingencies - Aircraft Purchase Commitments By Aircraft (Details) - Future aircraft purchase commitments - aircraft	Sep. 30, 2018	Jun. 30, 2018
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	323
A220-100 (formerly called CS100)
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	75
A321-200
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	64
A321-200neo
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	100
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	14
B-737-900ER
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	26
CRJ-900
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	19	20