Commitments and Contingencies - Firm Commitments and Options to Purchase Aircraft (Details)	Sep. 30, 2021 commitment
Airbus A321XLR
Long-term Purchase Commitment [Line Items]
Number of firm commitments	50
Scheduled Aircraft Deliveries
Last Three Months of 2021	0
2022	0
2023	0
After 2023	50
Airbus A321neo
Long-term Purchase Commitment [Line Items]
Number of firm commitments	70
Scheduled Aircraft Deliveries
Last Three Months of 2021	0
2022	0
2023	16
After 2023	54
Airbus A350
Long-term Purchase Commitment [Line Items]
Number of firm commitments	45
Scheduled Aircraft Deliveries
Last Three Months of 2021	0
2022	0
2023	0
After 2023	45
Boeing 737 MAX
Long-term Purchase Commitment [Line Items]
Number of firm commitments	371
Scheduled Aircraft Deliveries
Last Three Months of 2021	4
2022	48
2023	114
After 2023	205
Boeing 787
Long-term Purchase Commitment [Line Items]
Number of firm commitments	8
Scheduled Aircraft Deliveries
Last Three Months of 2021	1
2022	7
2023	0
After 2023	0