Commitments and Contingencies - Aircraft-Related Commitments and Financing Arrangements (Details) $ in Millions	Mar. 31, 2020 USD ($) aircraft aircraft_engine	Dec. 31, 2019 aircraft
Principal And Interest Commitments [Abstract]
Principal and interest commitments related to future secured debt, remainder of fiscal year	$ 7.8
Principal and interest commitments related to future secured debt, 2021	16.5
Principal and interest commitments related to future secured debt, 2022	13.0
Principal and interest commitments related to future secured debt, 2023	14.7
Principal and interest commitments related to future secured debt, 2024	14.3
Principal and interest commitments related to future secured debt, 2025 and beyond	$ 124.2
Airbus
Unrecorded Unconditional Purchase Obligation [Line Items]
Future aircraft to be received \| aircraft	131
Secured Debt
Interest Commitments
Interest commitments, remainder of 2020	$ 61.8
Interest commitments, 2021	74.7
Interest commitments, 2022	68.1
Interest commitments, 2023	57.6
Interest commitments, 2024	46.7
Interest commitments, 2025 and beyond	$ 126.8
2020 \| Airbus
Committed Expenditures
Number of aircraft with secured debt financing commitments scheduled for delivery \| aircraft	4
2020-2027 \| Airbus
Committed Expenditures
Number of aircraft without secured financing commitments scheduled for delivery \| aircraft	127
Aircraft and Related Flight Equipment
Committed Expenditures
Committed expenditures, remainder of 2020	$ 701.6
Committed expenditures, 2021	735.9
Committed expenditures, 2022	115.3
Committed expenditures, 2023	491.6
Committed expenditures, 2024	1,002.5
Committed expenditures, 2025 and beyond	3,605.4
Non-aircraft Related Commitments
Committed Expenditures
Committed expenditures, remainder of 2020	15.2
Committed expenditures, 2021	17.1
Committed expenditures, 2022	16.2
Committed expenditures, 2023	14.4
Committed expenditures, 2024	14.5
Committed expenditures, 2025 and beyond	$ 50.3
V2500 SelectTWO Engine \| 2020-2023
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of spare aircraft engines ordered \| aircraft_engine	1
PurePower PW1100G-JM Engine \| 2020-2023
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 \| aircraft		50
A-320-Neo \| Third Party Lessor
Unrecorded Unconditional Purchase Obligation [Line Items]
Future aircraft to be received \| aircraft	11
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	$ 2.6
Operating lease payments expected for aircraft to be delivered, 2021	21.3
Operating lease payments expected for aircraft to be delivered, 2022	36.8
Operating lease payments expected for aircraft to be delivered, 2023	36.8
Operating lease payments expected for aircraft to be delivered, 2024	36.8
Operating lease payments expected for aircraft to be delivered, 2025 and beyond	$ 307.0
A320 and A321
Principal And Interest Commitments [Abstract]
Number of delivered aircraft with secured debt financing commitments \| aircraft	68

Aircraft Rent Commitments For Future Aircraft Deliveries

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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 Order Conversions

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Number of Aircraft Ordered

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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 with Secured Debt Financing Commitments Scheduled for Delivery

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Principal And Interest Commitments

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Principal And Interest Commitments Related To Future Secured Debt After Year Five

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Principal And Interest Commitments Related To Future Secured Debt, Due Remainder Of Fiscal Year

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 440
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=77896938&loc=d3e25336-109308

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Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing in the remainder of the fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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