Commitments and Contingencies - Aircraft-Related Commitments and Financing Arrangements (Details) $ in Millions	3 Months Ended
Mar. 31, 2018 USD ($) aircraft	Mar. 31, 2019 USD ($) aircraft_engine aircraft
Secured Debt [Member]
Interest Commitments
Interest commitments, remainder of 2019		$ 64.9
Interest commitments, 2020		77.4
Interest commitments, 2021		70.3
Interest commitments, 2022		63.4
Interest commitments, 2023		53.2
Interest commitments, 2024 and beyond		154.2
Principal and Interest Commitments
Principal and interest commitments related to future secured debt, remainder of 2019		2.2
Principal and interest commitments related to future secured debt, 2020		4.7
Principal and interest commitments related to future secured debt, 2021		4.7
Principal and interest commitments related to future secured debt, 2022		3.5
Principal and interest commitments related to future secured debt, 2023		3.6
Principal and interest commitments related to future secured debt, 2024 and beyond		$ 26.2
2019 [Member] \| Airbus [Member]
Aircraft rent commitments
Number of aircraft with secured debt financing commitments scheduled for delivery \| aircraft		1
2019-2021 [Member] [Member] \| Airbus [Member]
Aircraft rent commitments
Number of aircraft without secured financing commitments scheduled for delivery \| aircraft		43
Aircraft and Related Flight Equipment [Member]
Committed Expenditures
Committed expenditures, remainder of 2019		$ 466.6
Committed expenditures, 2020		865.7
Committed expenditures, 2021		775.7
Committed expenditures, 2022		17.6
Committed expenditures, 2023		8.3
Committed expenditures, 2024 and beyond		0.0
Face amount, commitment for future issuance		35.0
Fixed Maintenance Reserve Payments, Aircraft and Related Flight Equipment
Committed Expenditures
Committed expenditures, remainder of 2019		4.3
Committed expenditures, 2020		5.6
Committed expenditures, 2021		5.7
Committed expenditures, 2022		4.9
Committed expenditures, 2023		4.1
Committed expenditures, 2024 and beyond		8.8
Non-aircraft Related Commitments [Member]
Committed Expenditures
Committed expenditures, remainder of 2019		9.3
Committed expenditures, 2020		14.4
Committed expenditures, 2021		11.3
Committed expenditures, 2022		11.2
Committed expenditures, 2023		11.3
Committed expenditures, 2024 and beyond		$ 56.0
V2500 SelectTWO Engine [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of spare aircraft engines ordered \| aircraft_engine		2
PurePower PW1100G-JM Engine [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of spare aircraft engines ordered \| aircraft_engine		7
A-320-Neo [Member] \| Third Party Lessor [Member]
Aircraft rent commitments
Aircraft rent commitments, remainder of 2019		$ 3.0
Aircraft rent commitments, 2020		4.1
Aircraft rent commitments, 2021		4.1
Aircraft rent commitments, 2022		4.1
Aircraft rent commitments, 2023		4.1
Aircraft rent commitments, 2024 and beyond		$ 29.4
Airbus A319 [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Purchase price	$ 285.0
Airbus A319 [Member] \| Capital Lease Obligations [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of aircrafts purchased \| aircraft	14
A320 and A321 [Member]
Aircraft rent commitments
Number of delivered aircraft with secured debt financing commitments \| aircraft		61
A320 [Member] \| 2019 [Member] \| Equipment Notes, 2017-1 [Member]
Aircraft rent commitments
Number of aircraft with secured debt financing commitments scheduled for delivery \| aircraft		1

Face/par value in contract for future debt financing commitments

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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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