Commitments and Contingencies - Aircraft-Related Commitments and Financing Arrangements (Details) $ in Millions	Jun. 30, 2019 USD ($) aircraft_engine aircraft
Secured Debt [Member]
Interest Commitments
Interest commitments, remainder of 2019	$ 42.6
Interest commitments, 2020	78.5
Interest commitments, 2021	71.3
Interest commitments, 2022	64.3
Interest commitments, 2023	54.1
Interest commitments, 2024 and beyond	$ 157.9
2019 [Member] \| Airbus [Member]
Committed Expenditures
Number of aircraft with secured sale-leaseback financing commitments scheduled for delivery \| aircraft	6
2019-2021 [Member] [Member] \| Airbus [Member]
Committed Expenditures
Number of aircraft without secured financing commitments scheduled for delivery \| aircraft	37
Aircraft and Related Flight Equipment [Member]
Committed Expenditures
Committed expenditures, remainder of 2019	$ 388.5
Committed expenditures, 2020	858.0
Committed expenditures, 2021	776.1
Committed expenditures, 2022	17.6
Committed expenditures, 2023	8.3
Committed expenditures, 2024 and beyond	0.0
Non-aircraft Related Commitments [Member]
Committed Expenditures
Committed expenditures, remainder of 2019	6.9
Committed expenditures, 2020	17.4
Committed expenditures, 2021	14.7
Committed expenditures, 2022	13.9
Committed expenditures, 2023	12.7
Committed expenditures, 2024 and beyond	$ 60.0
V2500 SelectTWO Engine [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of spare aircraft engines ordered \| aircraft_engine	1
PurePower PW1100G-JM Engine [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of spare aircraft engines ordered \| aircraft_engine	6
A320 and A321 [Member]
Committed Expenditures
Number of delivered aircraft with secured debt financing commitments \| aircraft	62

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, After Year Five

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Number of Spare Aircraft Engines Ordered

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