Commitments and Contingencies - Aircraft-Related Commitments and Financing Arrangements (Details) $ in Millions	Mar. 31, 2021 USD ($) aircraft_Engine aircraft
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of aircraft to be delivered, remainder of fiscal year \| aircraft	6
Number of aircraft to be delivered, 2022 \| aircraft	17
Airbus
Unrecorded Unconditional Purchase Obligation [Line Items]
Future aircraft to be received \| aircraft	126
Aircraft-Related Secured Debt
Interest Commitments
Interest commitments, remainder of fiscal year	$ 59.4
Interest commitments, 2022	71.7
Interest commitments, 2023	61.0
Interest commitments, 2024	49.8
Interest commitments, 2025	42.4
Interest commitments, 2026 and thereafter	96.1
Non-Aircraft-Related Secured Debt, Unsecured Debt And Convertible Debt
Interest Commitments
Interest commitments, remainder of fiscal year	60.1
Interest commitments, 2022	77.3
Interest commitments, 2023	77.3
Interest commitments, 2024	77.3
Interest commitments, 2025	68.2
Interest commitments, 2026 and thereafter	$ 11.8
2021-2027 \| 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 fiscal year	$ 380.3
Committed expenditures, 2022	884.5
Committed expenditures, 2023	908.3
Committed expenditures, 2024	980.0
Committed expenditures, 2025	1,066.6
Committed expenditures, 2026 and thereafter	2,248.4
Non-aircraft Related Commitments
Committed Expenditures
Committed expenditures, remainder of fiscal year	20.7
Committed expenditures, 2022	21.4
Committed expenditures, 2023	18.9
Committed expenditures, 2024	16.1
Committed expenditures, 2025	16.7
Committed expenditures, 2026 and thereafter	$ 35.6
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 \| Third Party Lessor
Unrecorded Unconditional Purchase Obligation [Line Items]
Future aircraft to be received \| aircraft	8
A320 Family \| Third Party Lessor
Aircraft Rent Commitments For Future Aircraft Deliveries [Abstract]
Operating lease payments expected for aircraft to be delivered, remainder of fiscal year	$ 13.7
Operating lease payments expected for aircraft to be delivered, 2022	29.2
Operating lease payments expected for aircraft to be delivered, 2023	29.2
Operating lease payments expected for aircraft to be delivered, 2024	29.2
Operating lease payments expected for aircraft to be delivered, 2025	29.2
Operating lease payments expected for aircraft to be delivered, 2026 and thereafter	$ 220.3
A320 and A321
Aircraft Rent Commitments For Future Aircraft Deliveries [Abstract]
Number of delivered aircraft with secured debt financing commitments \| aircraft	72

Aircraft Rent Commitments For Future Aircraft Deliveries

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

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Long-Term Debt, Maturities, Repayments Of Interest And Fee Commitments, To Be Paid, Year One

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Long-Term Debt, Maturities, Repayments Of Interest And Fee Commitments, To Be Paid, Year Three

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Number Of Aircraft To Be Delivered, Remainder Of Fiscal Year

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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 Delivered Aircraft with Secured Debt Financing Commitments

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Unrecorded Unconditional Purchase Obligation, To Be Paid, After Year Four

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Amount of fixed and determinable portion of unrecorded unconditional purchase obligation to be paid in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 440
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121559207&loc=d3e25336-109308

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Amount of fixed and determinable portion of unrecorded unconditional purchase obligation to be paid in remainder of current fiscal year.

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