Leases, Commitments and Contingencies - Leases (Details) $ in Thousands		3 Months Ended			6 Months Ended		12 Months Ended
	Jan. 01, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($) aircraft	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) aircraft	Jun. 30, 2018 USD ($)	Dec. 31, 2016 item	Dec. 31, 2018 USD ($)
Lessee, Lease, Description [Line Items]
Number of aircraft leased | aircraft					92
Transition adjustment, net of tax								$ (13,141)
Operating lease right-of-use assets		$ 336,703			$ 336,703
Current maturities of lease liabilities		81,796			81,796
Noncurrent operating leases		$ 265,516			265,516
Operating leases					$ 43,100
Weighted-average remaining lease term: Operating leases		6 years 6 months			6 years 6 months
Weighted-average discount rate: Operating leases		6.40%			6.40%
Lease costs
Operating lease cost		$ 27,647		$ 45,425	$ 54,868	$ 98,258
Variable and short-term lease cost		1,346		1,392	2,821	2,811
Total lease cost		$ 28,993		$ 46,817	$ 57,689	$ 101,069
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			52
Payments for acquired aircraft			$ 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					9 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