Commitments and Contingencies - Schedule of Future Aircraft Operating Leases (Details)	1 Months Ended	3 Months Ended
	Jul. 31, 2015 aircraft seat	Mar. 31, 2018 aircraft	Dec. 31, 2018 aircraft_engine aircraft	Sep. 30, 2018 aircraft
Long-term Purchase Commitment [Line Items]
2019			16
2020			16
2021			18
Total future aircraft to be received			50
Airbus
Long-term Purchase Commitment [Line Items]
Number of delivered aircraft with secured debt financing commitments			60
Airbus | A320
Long-term Purchase Commitment [Line Items]
2019			2
2020
2021
Total future aircraft to be received			2
Number of aircrafts with increased seating capacity	40
Airbus | A320NEO
Long-term Purchase Commitment [Line Items]
2019			9
2020			16
2021			18
Total future aircraft to be received			43
Third Party Lessor | A320NEO
Long-term Purchase Commitment [Line Items]
2019			5
2020
Total future aircraft to be received			5
Aircraft order contract renegotiation | Airbus | Airbus A320
Long-term Purchase Commitment [Line Items]
Number of Aircraft Order Conversions			5
Number Of Aircraft Order Conversions, Delivered			3
2018 | Airbus
Long-term Purchase Commitment [Line Items]
Number of aircraft with secured debt financing commitments scheduled for delivery			2
2018-2021 [Member] | Airbus
Long-term Purchase Commitment [Line Items]
Number of aircraft without secured financing commitments scheduled for delivery			43
2019 | Airbus | A320NEO
Long-term Purchase Commitment [Line Items]
Future Aircraft To Be Received, Prior to Amended Agreement			14
Capital Lease Obligations | Airbus A319
Long-term Purchase Commitment [Line Items]
Property, Plant and Equipment, Number Of Aircraft Purchased		14
V2500 SelectOne Engine
Long-term Purchase Commitment [Line Items]
Number of spare aircraft engines ordered | aircraft_engine			2
PurePower PW1100G-JM Engine
Long-term Purchase Commitment [Line Items]
Number of spare aircraft engines ordered | aircraft_engine			8
Equipment Notes, 2017-1 [Member] | 2018 | A320
Long-term Purchase Commitment [Line Items]
Number of aircraft with secured debt financing commitments scheduled for delivery			2
Minimum | Airbus | A320
Long-term Purchase Commitment [Line Items]
Number of seats | seat	178
Maximum | Airbus | A320
Long-term Purchase Commitment [Line Items]
Number of seats | seat	182