Commitments and Contingencies - Aircraft-Related Commitments and Financing Arrangements (Details)		1 Months Ended	3 Months Ended
	Mar. 28, 2018 USD ($) aircraft	Jul. 31, 2015 aircraft seat	Mar. 31, 2018 USD ($) aircraft aircraft_engine	Mar. 31, 2017 USD ($) aircraft
Unrecorded Unconditional Purchase Obligation [Line Items]
Aircraft scheduled for delivery | aircraft			53
Capital lease assets within flight equipment	$ 236,700,000
Capital lease obligation	143,800,000
Number of engines ordered | aircraft_engine			6
Number of engines purchased | aircraft_engine			2
Principal and Interest Commitments
Number of Spare Engines Capitalized | aircraft			5
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			$ 56,500,000
Interest commitments, 2019			63,700,000
Interest commitments, 2020			57,900,000
Interest commitments, 2021			52,300,000
Interest commitments, 2022			46,700,000
Interest commitments, 2023 and beyond			163,300,000
Principal and Interest Commitments
Principal and interest commitments related to future secured debt, 2018			3,300,000
Principal and interest commitments related to future secured debt, 2019			16,100,000
Principal and interest commitments related to future secured debt, 2020			14,700,000
Principal and interest commitments related to future secured debt, 2021			11,700,000
Principal and interest commitments related to future secured debt, 2022			11,200,000
Principal and interest commitments related to future secured debt, 2023 and beyond			$ 108,700,000
Aircraft [Member]
Principal and Interest Commitments
Number of Aircraft Capitalized | aircraft			60
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			4
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			$ 445,700,000
Committed expenditures, 2019			600,700,000
Committed expenditures, 2020			821,300,000
Committed expenditures, 2021			785,100,000
Committed expenditures, 2022			16,800,000
Committed expenditures, 2023 and beyond			7,900,000
Fixed Maintenance Reserve Payments, Aircraft and Related Flight Equipment [Member]
Committed Expenditures
Committed expenditures, remainder of 2018			6,600,000
Committed expenditures, 2019			5,900,000
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			7,700,000
Committed expenditures, 2019			12,200,000
Committed expenditures, 2020			12,300,000
Committed expenditures, 2021			9,500,000
Committed expenditures, 2022			9,700,000
Committed expenditures, 2023 and beyond			$ 64,600,000
V2500 SelectTWO Engine [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of spare aircraft engines ordered | aircraft_engine			3
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] | 2019 [Member] | Airbus [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of Aircraft Order Conversions | aircraft			5
A-320-Neo [Member] | Airbus [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Aircraft scheduled for delivery | aircraft			43
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			52
A320 Family [Member]
Principal and Interest Commitments
Number of Aircraft Held | aircraft			118
A320 Family [Member] | Aircraft [Member]
Principal and Interest Commitments
Number of leased assets | aircraft			58
Airbus A319 [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Purchase price	$ 285,000,000.0
Airbus A319 [Member] | Aircraft [Member]
Principal and Interest Commitments
Number of Previously Leased Aircraft or Engine Purchased | aircraft			8
A320 [Member] | Airbus [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Aircraft scheduled for delivery | aircraft			10
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
Capital Lease Obligations [Member] | Airbus A319 [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of aircrafts purchased | aircraft	14