Property and Equipment (Schedule of Property and Equipment) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Property, Plant and Equipment, Net, by Type [Abstract]
Property and equipment, gross	$ 2,164,223	$ 2,003,988
Accumulated depreciation	(937,723)	(844,026)
Property and equipment, net	1,226,500	1,159,962
Flight Equipment [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property and equipment, gross	1,927,864	1,801,808
Ground equipment [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property and equipment, gross	55,548	53,523
facilities, leasehold improvements and office equipment [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property and equipment, gross	28,609	26,897
Construction in Progress [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property and equipment, gross	$ 152,202	$ 121,760