Commitments and Contingencies - Firm Commitments and Options to Purchase Aircraft (Details)	Sep. 30, 2019 commitment Aircraft
Scheduled Aircraft Deliveries
2020 | Aircraft	17
Airbus A350
Long-term Purchase Commitment [Line Items]
Number of firm commitments	45
Scheduled Aircraft Deliveries
Last Three Months of 2019	0
2020	0
After 2020	45
Boeing 737 MAX
Long-term Purchase Commitment [Line Items]
Number of firm commitments	171
Scheduled Aircraft Deliveries
Last Three Months of 2019	16
2020	28
After 2020	127
Boeing 777-300ER
Long-term Purchase Commitment [Line Items]
Number of firm commitments	4
Scheduled Aircraft Deliveries
Last Three Months of 2019	2
2020	2
After 2020	0
Boeing 787
Long-term Purchase Commitment [Line Items]
Number of firm commitments	18
Scheduled Aircraft Deliveries
Last Three Months of 2019	2
2020	15
After 2020	1
Embraer E175
Long-term Purchase Commitment [Line Items]
Number of firm commitments	29
Scheduled Aircraft Deliveries
Last Three Months of 2019	9
2020	20
After 2020	0