Commitments and Contingencies - Aircraft-Related Commitments and Financing Arrangements (Details) ft² in Thousands, $ in Millions	1 Months Ended
	Sep. 30, 2015 a ft² extension_option	Jul. 31, 2015 aircraft seat	Dec. 31, 2017 USD ($) aircraft_engine aircraft	Nov. 30, 2017 USD ($)
Unrecorded Unconditional Purchase Obligation [Line Items]
Future Aircraft To Be Received | aircraft			59
Hangar Facility | Operating Lease
Principal and Interest Commitments
Area of Land | a	10
Real estate property (in square feet) | ft²	126
Operating leases, term	30 years
Number of renewal options | extension_option	2
Aircraft operating leases, renewal term	10 years
Airbus
Committed Expenditures
Number of delivered aircraft with secured debt financing commitments | aircraft			46
Secured Debt
Interest Commitments
Interest commitments, 2018			$ 60.8
Interest commitments, 2019			56.2
Interest commitments, 2020			51.0
Interest commitments, 2021			45.8
Interest commitments, 2022			40.7
Interest commitments, 2023 and beyond			133.3
Principal and Interest Commitments
Principal and interest commitments, 2018			19.6
Principal and interest commitments, 2019			39.4
Principal and interest commitments, 2020			35.8
Principal and interest commitments, 2021			30.3
Principal and interest commitments, 2022			29.0
Principal and interest commitments, 2023 and beyond			$ 301.4
Enhanced Equipment Trust Certificate
Committed Expenditures
Face amount, commitment for future issuance				$ 420.5
Number of delivered aircraft with secured debt financing commitments | aircraft			2
Aircraft and Related Flight Equipment
Principal and Interest Commitments
Estimated future project construction costs			$ 0.6
2018 | Airbus
Committed Expenditures
Number of aircraft with secured debt financing commitments scheduled for delivery | aircraft			10
2018 | Enhanced Equipment Trust Certificate | Airbus
Committed Expenditures
Number of aircraft with secured debt financing commitments scheduled for delivery | aircraft			10
2019-2021 | Airbus
Committed Expenditures
Number of aircraft without secured financing commitments scheduled for delivery | aircraft			49
Aircraft and Related Flight Equipment
Committed Expenditures
Committed expenditures, 2018			$ 528.4
Committed expenditures, 2019			774.8
Committed expenditures, 2020			820.9
Committed expenditures, 2021			785.1
Committed expenditures, 2022			16.8
Committed expenditures, 2023 and beyond			7.9
Aircraft and Related Flight Equipment | Enhanced Equipment Trust Certificate
Committed Expenditures
Face amount, commitment for future issuance			356.6
Non-aircraft Related Commitments
Committed Expenditures
Committed expenditures, 2018			5.9
Committed expenditures, 2019			1.8
Committed expenditures, 2020			1.3
Committed expenditures, 2021			0.7
Committed expenditures, 2022			0.4
Committed expenditures, 2023 and beyond			$ 0.0
V2500 SelectOne Engine
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of spare aircraft engines ordered | aircraft_engine			4
PurePower PW1100G-JM Engine
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of spare aircraft engines ordered | aircraft_engine			9
A320NEO | Airbus
Unrecorded Unconditional Purchase Obligation [Line Items]
Future Aircraft To Be Received | aircraft			48
A320NEO | 2018 | Airbus
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of Aircraft Order Conversions | aircraft			4
A320NEO | 2019 | Airbus
Unrecorded Unconditional Purchase Obligation [Line Items]
Future Aircraft To Be Received | aircraft			14
A320 | Airbus
Unrecorded Unconditional Purchase Obligation [Line Items]
Future Aircraft To Be Received | aircraft			6
Principal and Interest Commitments
Number of aircrafts with increased seating capacity | aircraft		40
A320 | Airbus | Minimum
Principal and Interest Commitments
Number of seats | seat		178
A320 | Airbus | Maximum
Principal and Interest Commitments
Number of seats | seat		182
Aircraft order contract renegotiation | A320NEO | 2019 | Airbus
Unrecorded Unconditional Purchase Obligation [Line Items]
Future Aircraft To Be Received | aircraft			2
Aircraft order contract renegotiation | Airbus A320 | 2018 | Airbus
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of Aircraft Order Conversions | aircraft			2
Aircraft order contract renegotiation | Airbus A320 | 2019 | Airbus
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of Aircraft Order Conversions | aircraft			5