Commitments and Contingencies - Aircraft-Related Commitments and Financing Arrangements (Details) $ in Millions	Dec. 31, 2020 USD ($) aircraft_Engine aircraft	Dec. 31, 2019 aircraft
Interest Commitments
Interest commitments, 2021	$ 162.5
Interest commitments, 2022	148.7
Interest commitments, 2023	138.1
Interest commitments, 2024	126.8
Interest commitments, 2025	110.0
Interest commitments, 2026 and beyond	103.0
Aircraft and Related Flight Equipment
Committed Expenditures
Committed expenditures, 2021	415.7
Committed expenditures, 2022	849.1
Committed expenditures, 2023	676.0
Committed expenditures, 2024	1,001.6
Committed expenditures, 2025	1,209.1
Committed expenditures, 2026 and beyond	2,367.8
Non-aircraft Related Commitments
Committed Expenditures
Committed expenditures, 2021	17.8
Committed expenditures, 2022	15.9
Committed expenditures, 2023	14.1
Committed expenditures, 2024	14.5
Committed expenditures, 2025	15.0
Committed expenditures, 2026 and beyond	$ 35.3
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
A320 Family
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of aircraft ordered \| aircraft		100
Number of aircraft order conversions \| aircraft		50
Airbus
Unrecorded Unconditional Purchase Obligation [Line Items]
Future aircraft to be received \| aircraft	126
Airbus \| 2021-2027
Committed Expenditures
Number of aircraft without secured financing commitments scheduled for delivery \| aircraft	126
Third Party Lessor \| A320NEO
Unrecorded Unconditional Purchase Obligation [Line Items]
Future aircraft to be received \| aircraft	10
Aircraft Rent Commitments [Abstract]
Aircraft rent commitments, 2021	$ 18.1
Aircraft rent commitments, 2022	34.2
Aircraft rent commitments, 2023	34.2
Aircraft rent commitments, 2024	34.2
Aircraft rent commitments, 2025	34.2
Aircraft rent commitments, 2026 and beyond	$ 255.5

Aircraft Rent Commitments [Abstract]

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Future Aircraft To Be Received

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Long-Term Debt, Maturities, Repayments Of Interest And Fee Commitments [Abstract]

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Number of Aircraft without Secured Financing Commitments Scheduled for Delivery

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Number of Spare Aircraft Engines Ordered

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Number of Aircraft Order Conversions

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