Summary of Significant Accounting Policies - Maintenance Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Deferred costs for heavy maintenance	$ 75,200	$ 176,000	$ 190,500
Deferred heavy maintenance, gross	570,600	504,200
Accumulated heavy maintenance amortization	680,230	492,447
Airframe and Engine Maintenance Costs [Abstract]
Total maintenance, materials and repairs	111,227	143,575	129,078
Heavy maintenance and major overhaul
Property, Plant and Equipment [Line Items]
Amortization of heavy maintenance	88,927	63,364	41,286
Heavy Maintenance
Property, Plant and Equipment [Line Items]
Accumulated heavy maintenance amortization	222,700	142,600
Flight hour-based maintenance expense
Airframe and Engine Maintenance Costs [Abstract]
Total maintenance, materials and repairs	52,092	78,253	68,039
Non-flight hour-based maintenance expense
Airframe and Engine Maintenance Costs [Abstract]
Total maintenance, materials and repairs	$ 59,135	$ 65,322	$ 61,039