Commitments and Contingencies - Aircraft-Related Commitments and Financing Arrangements (Details) $ in Millions	Sep. 30, 2023 USD ($) aircraft aircraft_engine	Dec. 31, 2022	Sep. 30, 2022
Aircraft-Related Secured Debt
Interest Commitments
Interest commitments, remainder of fiscal year	$ 15.4
Interest commitments, 2024	53.3
Interest commitments, 2025	45.8
Interest commitments, 2026	38.3
Interest commitments, 2027	30.1
Interest commitments, 2028 and thereafter	$ 60.2
Secured Debt | 8.00% Senior Secured Notes Due 2025
Interest Commitments
Stated interest rate percentage	8.00%	8.00%	8.00%
Non-Aircraft-Related Secured Debt, Unsecured Debt And Convertible Debt
Interest Commitments
Interest commitments, remainder of fiscal year	$ 25.7
Interest commitments, 2024	96.4
Interest commitments, 2025	89.4
Interest commitments, 2026	5.9
Interest commitments, 2027	3.4
Interest commitments, 2028 and thereafter	$ 10.5
A320 and A321
Aircraft Rent Commitments For Future Aircraft Deliveries
Number of delivered aircraft with secured debt financing commitments | aircraft	73
Aircraft and Related Flight Equipment
Committed Expenditures
Committed expenditures, remainder of fiscal year	$ 120.3
Committed expenditures, 2024	456.0
Committed expenditures, 2025	1,018.6
Committed expenditures, 2026	1,034.3
Committed expenditures, 2027	1,100.0
Committed expenditures, 2028 and thereafter	1,959.1
Non-aircraft Related Commitments
Committed Expenditures
Committed expenditures, remainder of fiscal year	35.9
Committed expenditures, 2024	21.7
Committed expenditures, 2025	19.9
Committed expenditures, 2026	17.1
Committed expenditures, 2027	17.1
Committed expenditures, 2028 and thereafter	$ 1.7
PurePower PW1100G-JM Engine | 2023 Through 2029
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of spare aircraft engines ordered | aircraft_engine	19
Airbus
Unrecorded Unconditional Purchase Obligation [Line Items]
Future aircraft to be received | aircraft	101
Number of aircraft to be delivered, remainder of fiscal year | aircraft	3
Number of aircraft to be delivered, next fiscal year | aircraft	7
Airbus | Through 2024
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of aircraft with secured debt financing commitments scheduled for delivery | aircraft	7
Airbus | 2023 Through 2029
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of aircraft without secured financing commitments scheduled for delivery | aircraft	94
Third Party Lessor | A320 Family
Aircraft Rent Commitments For Future Aircraft Deliveries
Operating lease payments expected for aircraft to be delivered, remainder of fiscal year	$ 4.7
Operating lease payments expected for aircraft to be delivered, 2024	90.3
Operating lease payments expected for aircraft to be delivered, 2025	140.8
Operating lease payments expected for aircraft to be delivered, 2026	143.2
Operating lease payments expected for aircraft to be delivered, 2027	143.2
Operating lease payments expected for aircraft to be delivered, 2028 and thereafter	$ 1,196.1
Third Party Lessor | 2023 Through 2025 | A321neos
Unrecorded Unconditional Purchase Obligation [Line Items]
Future aircraft to be received | aircraft	24