Commitments and Contingencies - Firm Commitments and Options to Purchase Aircraft (Details)	Dec. 31, 2018 commitment
Airbus A350
Long-term Purchase Commitment [Line Items]
Number aircraft under purchase agreements	45
Boeing 737 MAX
Long-term Purchase Commitment [Line Items]
Number aircraft under purchase agreements	175
Boeing 777-300ER
Long-term Purchase Commitment [Line Items]
Number aircraft under purchase agreements	4
Boeing 787
Long-term Purchase Commitment [Line Items]
Number aircraft under purchase agreements	24
Embraer E175
Long-term Purchase Commitment [Line Items]
Number aircraft under purchase agreements	25