Commitments and Contingencies - Aircraft-Related Commitments and Financing Arrangements (Details)		1 Months Ended	6 Months Ended
	Mar. 28, 2018 USD ($) aircraft	Jul. 31, 2015 aircraft seat	Jun. 30, 2018 USD ($) aircraft_engine aircraft	Jun. 30, 2017 USD ($) aircraft
Unrecorded Unconditional Purchase Obligation [Line Items]
Capital lease assets within flight equipment	$ 236,700,000
Capital lease obligation	143,800,000
Number of engines ordered | aircraft_engine			6
Property, Plant And Equipment, Number Of Engines Sold | aircraft_engine			5
Number of engines purchased | aircraft_engine			4
Principal and Interest Commitments
Number of Spare Engines Capitalized | aircraft			6
Estimated Remaining Project Construction Costs			$ 0
Spare Engines [Member]
Principal and Interest Commitments
Number of leased assets | aircraft_engine			12
Secured Debt [Member]
Interest Commitments
Interest commitments, 2018			$ 40,300,000
Interest commitments, 2019			73,400,000
Interest commitments, 2020			67,000,000
Interest commitments, 2021			60,700,000
Interest commitments, 2022			54,400,000
Interest commitments, 2023 and beyond			170,800,000
Principal and Interest Commitments
Principal and interest commitments related to future secured debt, 2018			2,700,000
Principal and interest commitments related to future secured debt, 2019			4,400,000
Principal and interest commitments related to future secured debt, 2020			4,000,000
Principal and interest commitments related to future secured debt, 2021			3,800,000
Principal and interest commitments related to future secured debt, 2022			3,600,000
Principal and interest commitments related to future secured debt, 2023 and beyond			$ 13,100,000
Aircraft [Member]
Principal and Interest Commitments
Number of Aircraft Capitalized | aircraft			75
Number of Previously Leased Aircraft or Engine Purchased | aircraft			22
Operating Leases of Lessee, Number of Assets Leased, Affected by Variable Rate Basis | aircraft			1
2018 [Member] | Airbus [Member]
Committed Expenditures
Number of aircraft with secured debt financing commitments scheduled for delivery | aircraft			3
2019-2021 [Member] [Member] | Airbus [Member]
Committed Expenditures
Number of aircraft without secured financing commitments scheduled for delivery | aircraft			49
Aircraft and Related Flight Equipment [Member]
Committed Expenditures
Committed expenditures, remainder of 2018			$ 345,600,000
Committed expenditures, 2019			600,700,000
Committed expenditures, 2020			821,600,000
Committed expenditures, 2021			785,100,000
Committed expenditures, 2022			16,800,000
Committed expenditures, 2023 and beyond			7,900,000
Aircraft and Related Flight Equipment [Member] | Enhanced Equipment Trust Certificate [Member]
Committed Expenditures
Face amount, commitment for future issuance			117,000,000
Fixed Maintenance Reserve Payments, Aircraft and Related Flight Equipment [Member]
Committed Expenditures
Committed expenditures, remainder of 2018			3,700,000
Committed expenditures, 2019			5,800,000
Committed expenditures, 2020			5,600,000
Committed expenditures, 2021			5,700,000
Committed expenditures, 2022			4,900,000
Committed expenditures, 2023 and beyond			12,900,000
Non-aircraft Related Commitments [Member]
Committed Expenditures
Committed expenditures, remainder of 2018			5,300,000
Committed expenditures, 2019			12,500,000
Committed expenditures, 2020			12,500,000
Committed expenditures, 2021			9,800,000
Committed expenditures, 2022			9,900,000
Committed expenditures, 2023 and beyond			65,800,000
V2500 SelectOne Engine [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Proceeds from Sale of Other Property, Plant, and Equipment			9,500,000
Gain (Loss) on Sale of Properties			$ 4,400,000
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			9
Spare Engines [Member]
Principal and Interest Commitments
Number of Previously Leased Aircraft or Engine Purchased | aircraft			1
Spare Engines [Member] | Spare Engines [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Purchase price				$ 8,100,000
Principal and Interest Commitments
Number of Previously Leased Aircraft or Engine Purchased | aircraft				1
Airbus A320 [Member] | Aircraft order contract renegotiation [Member] | Airbus [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of Aircraft Order Conversions | aircraft			5
A-320-Neo [Member] | 2019 [Member] | Airbus [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Aircraft scheduled for delivery | aircraft			14
A320 and A321 [Member] | Airbus [Member]
Committed Expenditures
Number of delivered aircraft with secured debt financing commitments | aircraft			53
A320 Family [Member]
Principal and Interest Commitments
Number of Aircraft Held | aircraft			119
A320 Family [Member] | Aircraft [Member]
Principal and Interest Commitments
Number of leased assets | aircraft			44
Airbus A319 [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Purchase price	$ 285,000,000
A320 [Member] | Airbus [Member]
Principal and Interest Commitments
Number of aircrafts with increased seating capacity | aircraft		40
A320 [Member] | Airbus [Member] | Minimum [Member]
Principal and Interest Commitments
Number of seats | seat		178
A320 [Member] | Airbus [Member] | Maximum [Member]
Principal and Interest Commitments
Number of seats | seat		182
A320 [Member] | 2018 [Member] | Equipment Notes, 2017-1 [Member]
Committed Expenditures
Number of aircraft with secured debt financing commitments scheduled for delivery | aircraft			3
Capital Lease Obligations [Member] | Airbus A319 [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of aircrafts purchased | aircraft	14