Significant Accounting Policies - Estimated Useful Lives of Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2018
Aircraft and related rotable parts | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life	25 years
Aircraft and related rotable parts | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life	30 years
Aircraft seats | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life	10 years
Aircraft seats | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life	15 years
Buildings | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life	25 years
Buildings | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life	45 years
Other property and equipment | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life	3 years
Other property and equipment | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life	15 years
Computer software | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life	5 years
Computer software | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life	15 years
Building improvements | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life	1 year
Building improvements | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life	40 years