Summary of Significant Accounting Policies - Maintenance Costs (Details) - USD ($) $ in Thousands	3 Months Ended				12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Deferred costs for heavy maintenance					$ 190,500	$ 78,200	$ 35,400
Deferred heavy maintenance, gross	$ 260,500				366,300	260,500
Accumulated heavy maintenance amortization	207,808				332,864	207,808	122,509
Airframe and Engine Maintenance Costs [Abstract]
Flight hour-based maintenance expense					68,039	54,802	48,471
Non-flight hour-based maintenance expense					61,039	55,637	50,116
Total maintenance, materials and repairs	28,966	$ 26,176	$ 28,985	$ 26,312	129,078	110,439	98,587
Heavy maintenance and major overhaul
Property, Plant and Equipment [Line Items]
Amortization of heavy maintenance					41,286	53,855	$ 43,811
Heavy Maintenance
Property, Plant and Equipment [Line Items]
Accumulated heavy maintenance amortization	$ 160,700				$ 117,300	$ 160,700