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	Jan. 30, 2017 aircraft	Jan. 01, 2017 USD ($)	Dec. 31, 2016 USD ($) aircraft aircraft_engine	Apr. 27, 2016 aircraft	Dec. 31, 2015 aircraft	Aug. 31, 2015 USD ($) aircraft
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
Estimated future project construction costs					$ 7.3
Airbus
Committed Expenditures
Number of delivered aircraft with secured debt financing commitments | aircraft					29
Secured Debt
Interest Commitments
Interest commitments, 2017					$ 42.2
Interest commitments, 2018					38.2
Interest commitments, 2019					34.6
Interest commitments, 2020					30.9
Interest commitments, 2021					27.4
Interest commitments, 2022 and beyond					96.8
Aircraft and Related Flight Equipment
Principal and Interest Commitments
Estimated future project construction costs					$ 2.6
Bank Debt | Airbus
Committed Expenditures
Number of aircraft with secured debt financing commitments scheduled for delivery | aircraft					3
Enhanced Equipment Trust Certificate and Bank Debt | Secured Debt
Principal and Interest Commitments
Principal and interest commitments, 2017					$ 10.9
Principal and interest commitments, 2018					13.0
Principal and interest commitments, 2019					13.7
Principal and interest commitments, 2020					13.2
Principal and interest commitments, 2021					13.3
Principal and interest commitments, 2022 and beyond					$ 119.1
2016 | Airbus
Committed Expenditures
Number of aircraft with secured debt financing commitments scheduled for delivery | aircraft					4
2016 | Subsequent Event | Airbus
Committed Expenditures
Number of aircraft with secured debt financing commitments scheduled for delivery | aircraft			1
2017-2021 | Airbus
Committed Expenditures
Number of aircraft without secured financing commitments scheduled for delivery | aircraft					72
Aircraft and Related Flight Equipment
Committed Expenditures
Committed expenditures, 2017					$ 660.3
Committed expenditures, 2018					649.8
Committed expenditures, 2019					678.3
Committed expenditures, 2020					824.4
Committed expenditures, 2021					777.9
Committed expenditures, 2022 and beyond					24.6
Face amount, commitment for future issuance					144.5
Aircraft and Related Flight Equipment | Enhanced Equipment Trust Certificate
Committed Expenditures
Face amount, commitment for future issuance					38.5			$ 576.6
Aircraft and Related Flight Equipment | Subsequent Event
Committed Expenditures
Face amount, commitment for future issuance				$ 39.0
Non-aircraft Related Commitments
Committed Expenditures
Committed expenditures, 2017					5.7
Committed expenditures, 2018					3.9
Committed expenditures, 2019					0.3
Committed expenditures, 2020					0.3
Committed expenditures, 2021					0.1
Committed expenditures, 2022 and beyond					$ 0.0
V2500 SelectOne Engine
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of spare aircraft engines ordered | aircraft_engine					3
PurePower PW1100G-JM Engine
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of spare aircraft engines ordered | aircraft_engine					9
A320 and A321 | Airbus
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of Aircraft Order Conversions | aircraft						10
A320 and A321 | Enhanced Equipment Trust Certificate | Airbus
Committed Expenditures
Number of aircraft with secured debt financing commitments scheduled for delivery | aircraft							1
A320 | Airbus
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
A320 | Enhanced Equipment Trust Certificate | Airbus
Committed Expenditures
Number of aircraft with secured debt financing commitments scheduled for delivery | aircraft								3
Number of delivered aircraft with secured debt financing commitments | aircraft					3