Commitments and Contingencies - Commitments (Details) ft² in Thousands, $ in Millions	1 Months Ended			3 Months Ended
	Feb. 29, 2016 aircraft	Sep. 30, 2015 ft² a extension_option aircraft	Jul. 31, 2015 USD ($) aircraft seat	Mar. 31, 2016 USD ($) aircraft_engine aircraft	Apr. 05, 2016 USD ($)	Aug. 31, 2015 USD ($)
Aircraft [Member]
Long-term Debt, Fiscal Year Maturity [Abstract]
Operating leases, lessee, number of leased assets | aircraft				59
Spare Engines [Member]
Long-term Debt, Fiscal Year Maturity [Abstract]
Operating leases, lessee, number of leased assets | aircraft_engine				11
Hangar Facility [Member] | Operating Lease [Member]
Long-term Debt, Fiscal Year Maturity [Abstract]
Area of land, hangar site | a		10
Area of hangar facility | ft²		126
Operating lease, initial term		30 years
Operating lease, number of extension options | extension_option		2
Operating lease, extension term		10 years
Estimated future project costs				$ 32.0
Aircraft [Member]
Long-term Debt, Fiscal Year Maturity [Abstract]
Number of delivered aircraft with secured debt financing commitments | aircraft				4
Number of Previously Leased Aircraft Purchased | aircraft	2
Property and equipment, useful life				25 years
Property and equipment, salvage value percentage				10.00%
Airbus A320 [Member]
Long-term Debt, Fiscal Year Maturity [Abstract]
Number of aircraft held | aircraft				83
2016 [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of aircraft with secured debt financing commitments scheduled for delivery | aircraft				8
Airbus [Member]
Long-term Debt, Fiscal Year Maturity [Abstract]
Costs to reconfigure aircraft seating			$ 0.6
Airbus [Member] | A320 [Member]
Long-term Debt, Fiscal Year Maturity [Abstract]
Number of aircraft reconfigured for increased seating capacity | aircraft			40
Airbus [Member] | A320 [Member] | Minimum [Member]
Long-term Debt, Fiscal Year Maturity [Abstract]
Number of aircraft seats | seat			178
Airbus [Member] | A320 [Member] | Maximum [Member]
Long-term Debt, Fiscal Year Maturity [Abstract]
Number of aircraft seats | seat			182
Airbus [Member] | 2015-2021 [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of Aircraft without Secured Financing Commitments Scheduled for Delivery (in aircraft) | aircraft		75
Third Party Lessor [Member] | 2016 [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of Direct Leased Aircraft Ordered From A Third Party | aircraft				5
Aircraft and Related Flight Equipment [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Unrecorded Unconditional Purchase Obligation, Due in Remainder of Fiscal Year				$ 405.0
Unrecorded Unconditional Purchase Obligation, Due in 2017				767.0
Unrecorded Unconditional Purchase Obligation, Due in 2018				621.0
Unrecorded Unconditional Purchase Obligation, Due in 2019				704.0
Unrecorded Unconditional Purchase Obligation, Due in 2020				706.0
Unrecorded Unconditional Purchase Obligation, Due in 2021 and beyond				812.0
Aircraft with Increased Seating Capacity [Member] | Airbus [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Unrecorded Unconditional Purchase Obligation, Due in Remainder of Fiscal Year				13.3
Unrecorded Unconditional Purchase Obligation, Due in 2017				2.5
Unrecorded Unconditional Purchase Obligation, Due after Two Years				$ 0.0
V2500 SelectOne Engine [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of spare aircraft engines ordered (in aircraft engine) | aircraft_engine				4
PurePower PW1100G-JM Engine [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of spare aircraft engines ordered (in aircraft engine) | aircraft_engine				9
Enhanced Equipment Trust Certificate [Member] | Aircraft and Related Flight Equipment [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Debt financing commitments						$ 576.6
Secured Debt [Member]
Long-term Debt, Fiscal Year Maturity [Abstract]
remainder of 2016				$ 27.7
2017				52.9
2018				40.9
2019				37.2
2020				34.3
2021 and thereafter				$ 315.9
Subsequent Event [Member] | Enhanced Equipment Trust Certificate [Member] | Aircraft and Related Flight Equipment [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Debt financing commitments					$ 308.5