Commitments and Contingencies - Aircraft-Related Commitments and Financing Arrangements (Details) $ in Millions	1 Months Ended		3 Months Ended
	Sep. 30, 2015 ft² a extension_option	Jul. 31, 2015 aircraft seat	Mar. 31, 2017 USD ($) aircraft_engine aircraft	Mar. 31, 2016 aircraft	Dec. 31, 2016 aircraft
Unrecorded Unconditional Purchase Obligation [Line Items]
Aircraft scheduled for delivery in 2018 | aircraft			73
Spare Engines [Member]
Principal and Interest Commitments
Number of leased assets | aircraft_engine			10
Aircraft [Member]
Principal and Interest Commitments
Number of leased assets | aircraft			61
Hangar Facility [Member] | Operating Lease [Member]
Principal and Interest Commitments
Estimated future project construction costs			$ 3.0
Area of Land | a	10
Real estate property (in square feet) | ft²	126,000
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			40.4
Interest commitments, 2018			42.4
Interest commitments, 2019			38.5
Interest commitments, 2020			34.6
Interest commitments, 2021			30.7
Interest commitments, 2022 and beyond			109.6
Principal and Interest Commitments
Principal and interest commitments, 2017			7.9
Principal and interest commitments, 2018			30.4
Principal and interest commitments, 2019			31.2
Principal and interest commitments, 2020			31.6
Principal and interest commitments, 2021			30.7
Principal and interest commitments, 2022 and beyond			$ 240.4
Aircraft [Member]
Committed Expenditures
Number of delivered aircraft with secured debt financing commitments | aircraft			3
Principal and Interest Commitments
Number of Delivered Leased Aircraft | aircraft			2
Number of previously leased aircraft purchased | 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			$ 1.4
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			65
Aircraft and Related Flight Equipment [Member]
Committed Expenditures
Committed expenditures, 2017			$ 575.6
Committed expenditures, 2018			521.9
Committed expenditures, 2019			765.1
Committed expenditures, 2020			830.6
Committed expenditures, 2021			785.8
Committed expenditures, 2022 and beyond			24.6
Face amount, commitment for future issuance			296.0
Non-aircraft Related Commitments [Member]
Committed Expenditures
Committed expenditures, 2018			4.4
Committed expenditures, 2019			0.8
Committed expenditures, 2020			0.5
Committed expenditures, 2021			0.1
Committed expenditures, 2022 and beyond			$ 0.0
V2500 SelectOne Engine [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of spare aircraft engines ordered | aircraft_engine			4
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 aircraft purchased | aircraft			1
A-320-Neo [Member] | Airbus [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Aircraft scheduled for delivery in 2018 | aircraft			48
A-320-Neo [Member] | 2018 [Member] | Airbus [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Aircraft scheduled for delivery in 2018 | aircraft					4
A-320-Neo [Member] | 2019 [Member] | Airbus [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Aircraft scheduled for delivery in 2018 | aircraft			2
A320 and A321 [Member] | Airbus [Member]
Committed Expenditures
Number of delivered aircraft with secured debt financing commitments | aircraft			32
Airbus A320 [Member]
Principal and Interest Commitments
Number of Aircraft Held | aircraft			100
A320 [Member] | Airbus [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Aircraft scheduled for delivery in 2018 | aircraft			12
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
Aircraft order contract renegotiation [Member] | Airbus A320 [Member] | 2016 [Member] | Airbus [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Aircraft scheduled for delivery in 2018 | aircraft			2