Condensed Consolidated Financial Statements - Property and Equipment (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Forecast
Property and Equipment
Depreciation		$ 1.8
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
Property and Equipment
Depreciable Life	18 years		18 years
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
Residual Value (as a percent)	20.00%		30.00%
Ground equipment
Property and Equipment
Depreciable Life	10 years
Residual Value (as a percent)	0.00%
Office equipment
Property and Equipment
Depreciable Life	7 years
Residual Value (as a percent)	0.00%
Leasehold improvements
Property and Equipment
Depreciable Life	15 years
Residual Value (as a percent)	0.00%
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