Commitments and Contingencies - Aircraft-Related Commitments and Financing Arrangements (Details) ft² in Thousands, $ in Millions	1 Months Ended		6 Months Ended
	Sep. 30, 2015 ft² a extension_option	Jul. 31, 2015 aircraft seat	Jun. 30, 2017 USD ($) aircraft_engine aircraft
Unrecorded Unconditional Purchase Obligation [Line Items]
Aircraft scheduled for delivery | aircraft			69
Aircraft [Member]
Principal and Interest Commitments
Number of leased assets | aircraft			61
Spare Engines [Member]
Principal and Interest Commitments
Number of leased assets | aircraft_engine			10
Hangar Facility [Member] | Operating Lease [Member]
Principal and Interest Commitments
Estimated future project construction costs			$ 0.2
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
Secured Debt [Member]
Interest Commitments
Interest commitments, 2017			25.5
Interest commitments, 2018			47.5
Interest commitments, 2019			43.3
Interest commitments, 2020			39.0
Interest commitments, 2021			34.8
Interest commitments, 2022 and beyond			125.3
Principal and Interest Commitments
Principal and interest commitments, 2017			5.1
Principal and interest commitments, 2018			32.6
Principal and interest commitments, 2019			32.4
Principal and interest commitments, 2020			33.3
Principal and interest commitments, 2021			32.1
Principal and interest commitments, 2022 and beyond			$ 256.5
Aircraft [Member]
Committed Expenditures
Number of delivered aircraft with secured debt financing commitments | aircraft			7
Principal and Interest Commitments
Number of Delivered Leased Aircraft | aircraft			2
Depreciable lives			25 years
Estimated residual values			10.00%
Aircraft and Related Flight Equipment [Member]
Principal and Interest Commitments
Estimated future project construction costs			$ 1.0
2016 [Member] | Airbus [Member]
Committed Expenditures
Number of aircraft with secured debt financing commitments scheduled for delivery | aircraft			8
2017-2021 [Member] | Airbus [Member]
Committed Expenditures
Number of aircraft without secured financing commitments scheduled for delivery | aircraft			61
Aircraft and Related Flight Equipment [Member]
Committed Expenditures
Committed expenditures, remainder of 2017			$ 413.2
Committed expenditures, 2018			528.4
Committed expenditures, 2019			764.4
Committed expenditures, 2020			829.8
Committed expenditures, 2021			784.8
Committed expenditures, 2022 and beyond			24.6
Face amount, commitment for future issuance			310.0
Fixed Maintenance Reserve Payments, Aircraft and Related Flight Equipment [Member]
Committed Expenditures
Committed expenditures, remainder of 2017			3.9
Committed expenditures, 2018			7.0
Committed expenditures, 2019			5.7
Committed expenditures, 2020			5.4
Committed expenditures, 2021			5.5
Committed expenditures, 2022 and beyond			17.7
Non-aircraft Related Commitments [Member]
Committed Expenditures
Committed expenditures, remainder of 2017			3.4
Committed expenditures, 2018			4.9
Committed expenditures, 2019			0.8
Committed expenditures, 2020			0.6
Committed expenditures, 2021			0.2
Committed expenditures, 2022 and beyond			$ 0.1
V2500 SelectOne Engine [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of spare aircraft engines ordered | aircraft_engine			6
PurePower PW1100G-JM Engine [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of spare aircraft engines ordered | aircraft_engine			9
Spare Engines [Member] | Spare Engines [Member]
Principal and Interest Commitments
Number of previously leased engines | aircraft_engine			1
A-320-Neo [Member] | Airbus [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Aircraft scheduled for delivery | aircraft			48
A-320-Neo [Member] | 2018 [Member] | Airbus [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Aircraft scheduled for delivery | aircraft			4
A-320-Neo [Member] | 2019 [Member] | Airbus [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Aircraft scheduled for delivery | aircraft			2
Airbus A320 [Member] | Aircraft order contract renegotiation [Member] | 2016 [Member] | Airbus [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Aircraft scheduled for delivery | aircraft			2
A320 and A321 [Member] | Airbus [Member]
Committed Expenditures
Number of delivered aircraft with secured debt financing commitments | aircraft			36
Airbus A320 [Member]
Principal and Interest Commitments
Number of Aircraft Held | aircraft			104
A320 [Member] | Airbus [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Aircraft scheduled for delivery | aircraft			9
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