Commitments and Contingencies - Aircraft Purchase Commitments By Aircraft (Details) - Future aircraft purchase commitments	Mar. 31, 2019 aircraft
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	302
A220-100
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	31
A220-300
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	50
A321-200
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	53
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	35
A350-900
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	12
B-737-900ER
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	9
CRJ-900
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	12