Commitments and Contingencies - Aircraft Purchase Commitments By Aircraft (Details) - Future aircraft purchase commitments	Jun. 30, 2018 aircraft
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	335
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	71
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	30
CRJ-900
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	20