Estimated Useful Lives of Property and Equipment (Detail)	12 Months Ended
	Dec. 31, 2017
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
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