Commitments and Contingencies - Aircraft-Related Commitments and Financing Arrangements (Details)	9 Months Ended
	Sep. 30, 2020 USD ($) aircraft_engine aircraft	Dec. 31, 2019 aircraft	Sep. 30, 2019 aircraft
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of aircraft delivered | aircraft	10
Number of aircraft to be delivered, remainder of fiscal year | aircraft	2
Number of aircraft to be delivered, 2021 | aircraft	6
Committed Expenditures
Number of aircraft with secured sale-leaseback financing commitments | aircraft	1		1
Principal And Interest Commitments [Abstract]
Principal and interest commitments related to future secured debt, remainder of fiscal year	$ 0
Principal and interest commitments related to future secured debt, 2021	4,600,000
Principal and interest commitments related to future secured debt, 2022	3,500,000
Principal and interest commitments related to future secured debt, 2023	3,600,000
Principal and interest commitments related to future secured debt, 2024	3,900,000
Principal and interest commitments related to future secured debt, 2025 and thereafter	$ 33,400,000
Airbus
Unrecorded Unconditional Purchase Obligation [Line Items]
Future aircraft to be received | aircraft	128
8.00% senior secured notes due in 2025
Interest Commitments
Interest commitments, remainder of 2020	$ 21,600,000
Interest commitments, 2021	77,600,000
Interest commitments, 2022	70,700,000
Interest commitments, 2023	60,100,000
Interest commitments, 2024	48,900,000
Interest commitments, 2025 and thereafter	135,300,000
Secured Debt, Unsecured Debt And Convertible Debt
Interest Commitments
Interest commitments, remainder of 2020	4,200,000
Interest commitments, 2021	83,300,000
Interest commitments, 2022	77,000,000.0
Interest commitments, 2023	77,000,000.0
Interest commitments, 2024	77,000,000.0
Interest commitments, 2025 and thereafter	$ 74,400,000
2020 | Airbus
Committed Expenditures
Number of aircraft with secured debt financing commitments scheduled for delivery | aircraft	1
2020-2027 | Airbus
Committed Expenditures
Number of aircraft without secured financing commitments scheduled for delivery | aircraft	126
Aircraft and Related Flight Equipment
Committed Expenditures
Committed expenditures, remainder of 2020	$ 79,700,000
Committed expenditures, 2021	357,200,000
Committed expenditures, 2022	606,300,000
Committed expenditures, 2023	707,800,000
Committed expenditures, 2024	1,073,000,000.0
Committed expenditures, 2025 and thereafter	3,763,400,000
Non-aircraft Related Commitments
Committed Expenditures
Committed expenditures, remainder of 2020	5,600,000
Committed expenditures, 2021	17,100,000
Committed expenditures, 2022	16,200,000
Committed expenditures, 2023	14,400,000
Committed expenditures, 2024	14,500,000
Committed expenditures, 2025 and thereafter	$ 50,300,000
V2500 SelectTWO Engine | 2021-2023
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of spare aircraft engines ordered | aircraft_engine	1
PurePower PW1100G-JM Engine | 2021-2023
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of spare aircraft engines ordered | aircraft_engine	2
A-320-Neo
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of aircraft ordered | aircraft		100
Number of additional aircraft available for purchase (up to) | aircraft		50
Committed Expenditures
Number of aircraft with secured debt financing commitments scheduled for delivery | aircraft	19
A-320-Neo | Third Party Lessor
Unrecorded Unconditional Purchase Obligation [Line Items]
Future aircraft to be received | aircraft	10
A320 Family | Third Party Lessor
Aircraft Rent Commitments For Future Aircraft Deliveries [Abstract]
Operating lease payments expected for aircraft to be delivered, remainder of 2020	$ 0
Operating lease payments expected for aircraft to be delivered, 2021	18,500,000
Operating lease payments expected for aircraft to be delivered, 2022	33,900,000
Operating lease payments expected for aircraft to be delivered, 2023	33,900,000
Operating lease payments expected for aircraft to be delivered, 2024	33,900,000
Operating lease payments expected for aircraft to be delivered, 2025 and thereafter	$ 286,900,000
A320 and A321
Aircraft Rent Commitments For Future Aircraft Deliveries [Abstract]
Number of delivered aircraft with secured debt financing commitments | aircraft	71