Summary of Significant Accounting Policies - Property and Equipment (Details) $ in Thousands	3 Months Ended				12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) aircraft_engine aircraft	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Property, Plant and Equipment [Line Items]
Number of Spare Engines Capitalized | aircraft					8
Number of Flight Simulators Capitalized | aircraft					1
Depreciation, Depletion and Amortization, Nonproduction [Abstract]
Depreciation					$ 129,412	$ 83,154	$ 54,171
Amortization of capitalized software					6,029	3,143	3,154
Total depreciation and amortization	$ 36,472	$ 36,840	$ 35,331	$ 31,509	$ 176,727	140,152	101,136
Spare engines
Property, Plant and Equipment [Line Items]
Number of aircraft under operating lease | aircraft_engine					12
Aircrafts, Major Spare Rotable Parts, Avionics, and Assemblies
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				$ 13,700	7,700
Capitalized software costs during the year					$ 12,000	1,400	4,100
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
Heavy maintenance and major overhaul
Depreciation, Depletion and Amortization, Nonproduction [Abstract]
Amortization of heavy maintenance					$ 41,286	$ 53,855	$ 43,811
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					14 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
A320 Family | Aircraft
Property, Plant and Equipment [Line Items]
Number of aircraft under operating lease | aircraft					46
A320 Family | Aircraft
Property, Plant and Equipment [Line Items]
Number of aircraft capitalized | aircraft					82