Nature of Operations and Summary of Significant Accounting Policies - Property and Equipment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Property and Equipment
Increased depreciation expense	$ 1.4
New aircraft
Property and Equipment
Depreciable Life		18 years
Residual Value (as a percent)		30.00%
New aircraft | Minimum
Property and Equipment
Depreciable Life	20 years
Residual Value (as a percent)	17.50%
New aircraft | Maximum
Property and Equipment
Depreciable Life	22 years
Residual Value (as a percent)	20.00%
Used Aircraft, rotable spares, and spare engines | Minimum
Property and Equipment
Residual Value (as a percent)	0.00%	0.00%
Used Aircraft, rotable spares, and spare engines | Maximum
Property and Equipment
Depreciable Life	18 years	18 years
Residual Value (as a percent)	20.00%	30.00%
Ground equipment
Property and Equipment
Residual Value (as a percent)	0.00%
Ground equipment | Maximum
Property and Equipment
Depreciable Life	10 years
Office equipment
Property and Equipment
Residual Value (as a percent)	0.00%
Office equipment | Maximum
Property and Equipment
Depreciable Life	7 years
Leasehold improvements
Property and Equipment
Residual Value (as a percent)	0.00%
Leasehold improvements | Maximum
Property and Equipment
Depreciable Life	15 years
Buildings
Property and Equipment
Residual Value (as a percent)	0.00%
Buildings | Minimum
Property and Equipment
Depreciable Life	20 years
Buildings | Maximum
Property and Equipment
Depreciable Life	39 years 6 months