Commitments and Contingencies - Aircraft Purchase Commitments by Aircraft Type (Details) - Future aircraft purchase commitments - aircraft	Dec. 31, 2018	Dec. 01, 2018	Nov. 30, 2018	Oct. 31, 2018	Jun. 30, 2018
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	330
A220-100
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	36	40
A220-300
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	50	50
A321-200
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	62
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		35	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	18
CRJ-900
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	15				20