Commitments and Contingencies - Aircraft Purchase Commitments By Aircraft (Details) - Future aircraft purchase commitments	Mar. 31, 2018 aircraft
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	338
A321-200
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	85
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	17
B-737-900ER
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	36
CS100
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	75