Summary of Significant Accounting Policies - Property and Equipment (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) aircraft_engine aircraft	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Property, Plant and Equipment [Line Items]
Number of Spare Engines Capitalized | aircraft	4
Number of Flight Simulators Capitalized | aircraft	1
Depreciation, Depletion and Amortization, Nonproduction [Abstract]
Depreciation	$ 86,297	$ 57,325	$ 30,797
Amortization of heavy maintenance	53,855	43,811	43,111
Total depreciation and amortization	140,152	101,136	73,908
Amortization of capitalized software costs	$ 3,100	3,200	3,100
Spare engines
Property, Plant and Equipment [Line Items]
Number of aircraft under operating lease | aircraft_engine	11
Aircrafts, Major Spare Rotable Parts, Avionics, and Assemblies [Member]
Property, Plant and Equipment [Line Items]
Residual value, percentage	10.00%
Aircraft
Property, Plant and Equipment [Line Items]
Depreciable lives used for the principal depreciable asset classifications	25 years
Spare rotables and flight assemblies | Minimum
Property, Plant and Equipment [Line Items]
Depreciable lives used for the principal depreciable asset classifications	7 years
Spare rotables and flight assemblies | Maximum
Property, Plant and Equipment [Line Items]
Depreciable lives used for the principal depreciable asset classifications	15 years
Other equipment and vehicles | Minimum
Property, Plant and Equipment [Line Items]
Depreciable lives used for the principal depreciable asset classifications	5 years
Other equipment and vehicles | Maximum
Property, Plant and Equipment [Line Items]
Depreciable lives used for the principal depreciable asset classifications	7 years
Internal use software
Depreciation, Depletion and Amortization, Nonproduction [Abstract]
Capitalized computer software, net	$ 7,700	9,400
Capitalized software costs during the year	$ 1,400	$ 4,100	$ 4,300
Internal use software | Minimum
Property, Plant and Equipment [Line Items]
Depreciable lives used for the principal depreciable asset classifications	3 years
Internal use software | Maximum
Property, Plant and Equipment [Line Items]
Depreciable lives used for the principal depreciable asset classifications	10 years
Spare engines | Minimum
Property, Plant and Equipment [Line Items]
Operating leases, term	2 years
Spare engines | Maximum
Property, Plant and Equipment [Line Items]
Operating leases, term	13 years
Buildings
Property, Plant and Equipment [Line Items]
Depreciable lives used for the principal depreciable asset classifications	30 years
Aircraft | Minimum
Property, Plant and Equipment [Line Items]
Operating leases, term	8 years
Aircraft | Maximum
Property, Plant and Equipment [Line Items]
Operating leases, term	18 years