Summary of Significant Accounting Policies - Maintenance Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Amortization of heavy maintenance	$ 53,855	$ 43,811	$ 43,111
Deferred costs for heavy maintenance	78,200	35,400	9,100
Deferred heavy maintenance, gross	260,500	238,300
Accumulated heavy maintenance amortization	207,808	122,509
Airframe and Engine Maintenance Costs [Abstract]
Flight hour-based maintenance expense	54,802	48,471	41,818
Non-flight hour-based maintenance expense	55,637	50,116	38,630
Total maintenance, materials and repairs	110,439	98,587	$ 80,448
Heavy Maintenance
Property, Plant and Equipment [Line Items]
Accumulated heavy maintenance amortization	$ 160,700	$ 162,800