Commitments and Contingencies - Aircraft Purchase Commitments By Aircraft (Details) - Future aircraft purchase commitments	Mar. 31, 2017 aircraft
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	245
B-737-900ER
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	55
A321-200
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	64
A330-300
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	1
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	25
CS100
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	75