Commitments and Contingencies - Aircraft-Related Commitments and Financing Arrangements (Details) ft² in Thousands, $ in Millions	1 Months Ended		3 Months Ended	9 Months Ended
	Sep. 30, 2015 ft² a extension_option	Jul. 31, 2015 aircraft seat	Sep. 30, 2017 USD ($) aircraft_engine aircraft	Sep. 30, 2017 USD ($) aircraft_engine aircraft	Mar. 31, 2017 aircraft
Unrecorded Unconditional Purchase Obligation [Line Items]
Aircraft scheduled for delivery | aircraft			65	65
Aircraft [Member]
Principal and Interest Commitments
Number of leased assets | aircraft			59	59
Spare Engines [Member]
Principal and Interest Commitments
Number of leased assets | aircraft_engine			11	11
Hangar Facility [Member] | Operating Lease [Member]
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
Secured Debt [Member]
Interest Commitments
Interest commitments, 2017			$ 19.5	$ 19.5
Interest commitments, 2018			52.9	52.9
Interest commitments, 2019			48.2	48.2
Interest commitments, 2020			43.6	43.6
Interest commitments, 2021			39.0	39.0
Interest commitments, 2022 and beyond			141.5	141.5
Principal and Interest Commitments
Principal and interest commitments related to future secured debt, 2017			0.0	0.0
Principal and interest commitments related to future secured debt, 2018			16.5	16.5
Principal and interest commitments related to future secured debt, 2019			16.4	16.4
Principal and interest commitments related to future secured debt, 2020			17.3	17.3
Principal and interest commitments related to future secured debt, 2021			16.2	16.2
Principal and interest commitments related to future secured debt, 2022 and beyond			$ 137.2	$ 137.2
Aircraft [Member]
Principal and Interest Commitments
Quantity of Delivered Aircraft with Secured Debt Financing Commitments | aircraft				11
Number of Delivered Leased Aircraft | aircraft				2
Number of Previously Leased Aircraft or Engine Purchased | aircraft_engine			1	1
Number of Leased Aircraft or Engine Returned | aircraft_engine				1
Depreciable lives				25 years
Estimated residual values			10.00%	10.00%
Aircraft and Related Flight Equipment [Member]
Principal and Interest Commitments
Estimated future project construction costs			$ 0.6	$ 0.6
2017 [Member] | Airbus [Member]
Committed Expenditures
Number of aircraft with secured debt financing commitments scheduled for delivery | aircraft			4	4
2017-2021 [Member] | Airbus [Member]
Committed Expenditures
Number of aircraft without secured financing commitments scheduled for delivery | aircraft			61	61
Aircraft and Related Flight Equipment [Member]
Committed Expenditures
Committed expenditures, remainder of 2017			$ 227.8	$ 227.8
Committed expenditures, 2018			528.4	528.4
Committed expenditures, 2019			773.7	773.7
Committed expenditures, 2020			820.5	820.5
Committed expenditures, 2021			784.8	784.8
Committed expenditures, 2022 and beyond			24.6	24.6
Face amount, commitment for future issuance			160.0	160.0
Fixed Maintenance Reserve Payments, Aircraft and Related Flight Equipment [Member]
Committed Expenditures
Committed expenditures, remainder of 2017			1.9	1.9
Committed expenditures, 2018			6.9	6.9
Committed expenditures, 2019			5.7	5.7
Committed expenditures, 2020			5.4	5.4
Committed expenditures, 2021			5.5	5.5
Committed expenditures, 2022 and beyond			17.7	17.7
Non-aircraft Related Commitments [Member]
Committed Expenditures
Committed expenditures, remainder of 2017			1.8	1.8
Committed expenditures, 2018			5.7	5.7
Committed expenditures, 2019			1.6	1.6
Committed expenditures, 2020			1.0	1.0
Committed expenditures, 2021			0.5	0.5
Committed expenditures, 2022 and beyond			$ 0.2	$ 0.2
V2500 SelectOne Engine [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of spare aircraft engines ordered | aircraft_engine			4	4
PurePower PW1100G-JM Engine [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of spare aircraft engines ordered | aircraft_engine			9	9
Spare Engines [Member] | Spare Engines [Member]
Principal and Interest Commitments
Number of Previously Leased Aircraft or Engine Purchased | aircraft_engine				1
Number of Spare Engines Purchased | aircraft_engine				2
Number of Delivered Leased Engines | aircraft_engine				1
Airbus A320 [Member] | Aircraft order contract renegotiation [Member] | 2017 [Member] | Airbus [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Aircraft scheduled for delivery | aircraft					2
A-320-Neo [Member] | Airbus [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Aircraft scheduled for delivery | aircraft			48	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] | Aircraft order contract renegotiation [Member] | 2019 [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			40	40
A320 Family [Member]
Principal and Interest Commitments
Number of Aircraft Held | aircraft			107	107
A320 [Member] | Airbus [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Aircraft scheduled for delivery | aircraft			8	8
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