Leases, Commitments and Contingencies - Leases (Details) $ in Thousands		3 Months Ended			9 Months Ended		12 Months Ended
	Jan. 01, 2019 USD ($)	Sep. 30, 2019 USD ($) aircraft	Mar. 31, 2019 USD ($) aircraft	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) aircraft	Sep. 30, 2018 USD ($)	Dec. 31, 2016 item
Lessee, Lease, Description [Line Items]
Number of aircraft leased | aircraft					91
Operating lease right-of-use assets		$ 326,700			$ 326,700
Current maturities of lease liabilities		87,472			87,472
Noncurrent operating leases		$ 254,258			254,258
Operating leases					$ 58,600
Weighted-average remaining lease term: Operating leases		6 years 9 months			6 years 9 months
Weighted-average discount rate: Operating leases		6.40%			6.40%
Lease costs
Operating lease cost		$ 25,173		$ 42,286	$ 80,041	$ 140,544
Variable and short-term lease cost		1,343		1,184	4,164	3,996
Sublease income		(357)			(357)
Total lease cost		$ 26,159		$ 43,470	$ 83,848	$ 144,540
CRJ 200
Lessee, Lease, Description [Line Items]
Number of seats on aircraft | item							50
Aircraft
Lessee, Lease, Description [Line Items]
Number of aircraft acquired | aircraft		4	52
Payments for acquired aircraft		$ 30,000	$ 111,700
Impairment adjustment on retained earnings	$ 13,100
Aircraft | Minimum
Lessee, Lease, Description [Line Items]
Remaining lease term					1 year
Aircraft | Maximum
Lessee, Lease, Description [Line Items]
Remaining lease term					10 years
Airport Facilities | Minimum
Lessee, Lease, Description [Line Items]
Remaining lease term					1 month
Airport Facilities | Maximum
Lessee, Lease, Description [Line Items]
Remaining lease term					37 years