Leases, Commitments and Contingencies - Leases (Details) $ in Thousands		3 Months Ended		12 Months Ended
	Jan. 01, 2019 USD ($)	Mar. 31, 2019 USD ($) aircraft	Mar. 31, 2018 USD ($)	Dec. 31, 2016 item	Dec. 31, 2018 USD ($)
Lessee, Lease, Description [Line Items]
Number of aircraft leased | aircraft		89
Transition adjustment, net of tax					$ (13,141)
Operating lease right-of-use assets		$ 341,898
Current maturities of lease liabilities		73,524
Noncurrent operating leases		275,088
Operating leases		$ 19,700
Weighted-average remaining lease term: Operating leases		7 years
Weighted-average discount rate: Operating leases		6.40%
Lease costs
Operating lease cost		$ 27,221	$ 52,833
Variable and short-term lease cost		1,475	1,419
Lease, Cost, Total		28,696	$ 54,252
Operating Lease Cost, Change		(25,612)
Variable and short-term lease cost, change		56
Total lease cost, change		$ (25,556)
Operating lease cost, change (as a percent)		(48.50%)
Variable and short-term lease cost, change (as a percent)		3.90%
Percentage of change in total lease cost.		(47.10%)
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