Commitments and Contingencies - Aircraft Contractual Purchase Commitments Narrative (Details) $ in Millions	9 Months Ended
	Sep. 30, 2016 USD ($) aircraft_engine aircraft	Apr. 27, 2016 aircraft	Aug. 31, 2015 USD ($)
A320 Family [Member]
Long-term Purchase Commitment [Line Items]
Number of aircraft held	89
Airbus [Member] | A320 and A321 [Member]
Long-term Purchase Commitment [Line Items]
Number of aircraft order conversions		10
Number of aircraft with secured debt financing commitments scheduled for delivery	2
Number of delivered aircraft with secured debt financing commitments	13
V2500 SelectOne Engine [Member]
Long-term Purchase Commitment [Line Items]
Number of spare aircraft engines ordered (in aircraft engine) | aircraft_engine	4
PurePower PW1100G-JM Engine [Member]
Long-term Purchase Commitment [Line Items]
Number of spare aircraft engines ordered (in aircraft engine) | aircraft_engine	9
Capital Addition Purchase Commitments [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due in Remainder of Fiscal Year | $	$ 104.0
Unrecorded Unconditional Purchase Obligation, Due in 2017 | $	659.0
Unrecorded Unconditional Purchase Obligation, Due in 2018 | $	650.0
Unrecorded Unconditional Purchase Obligation, Due in 2019 | $	679.0
Unrecorded Unconditional Purchase Obligation, Due in 2020 | $	823.0
Unrecorded Unconditional Purchase Obligation, Due in 2021 and beyond | $	$ 803.0
2016 [Member]
Long-term Purchase Commitment [Line Items]
Number of aircraft with secured debt financing commitments scheduled for delivery	1
2016 [Member] | Third Party Lessor [Member]
Long-term Purchase Commitment [Line Items]
Number of Direct Leased Aircraft Ordered From A Third Party	5
2017 [Member]
Long-term Purchase Commitment [Line Items]
Number of aircraft with secured debt financing commitments scheduled for delivery	1
2017-2021 [Member] | Airbus [Member]
Long-term Purchase Commitment [Line Items]
Number of Aircraft without Secured Financing Commitments Scheduled for Delivery	75
Enhanced Equipment Trust Certificate [Member] | Capital Addition Purchase Commitments [Member]
Long-term Purchase Commitment [Line Items]
Debt financing commitments | $			$ 576.6
Enhanced Equipment Trust Certificate [Member] | 2016 [Member] | Capital Addition Purchase Commitments [Member]
Long-term Purchase Commitment [Line Items]
Debt financing commitments | $	$ 38.7
Enhanced Equipment Trust Certificate [Member] | 2017 [Member] | Capital Addition Purchase Commitments [Member]
Long-term Purchase Commitment [Line Items]
Debt financing commitments | $	$ 38.5
Aircraft [Member]
Long-term Purchase Commitment [Line Items]
Number of delivered aircraft with secured debt financing commitments	10
Number of Previously Leased Aircraft Purchased	6
Property and equipment, useful life	25 years
Property and equipment, salvage value percentage	10.00%
Spare Engines [Member]
Long-term Purchase Commitment [Line Items]
Operating leases, lessee, number of leased assets | aircraft_engine	11
Aircraft [Member]
Long-term Purchase Commitment [Line Items]
Operating leases, lessee, number of leased assets	55