Summary of Significant Accounting Policies - Long-Lived Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Less: accumulated depreciation and amortization	$ (14,097)	$ (12,456)
Total property and equipment, net	26,563	24,375
Flight Equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 30,688	28,135
Estimated useful life	20-32 years
Flight Equipment | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life	32 years
Flight Equipment | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life	20 years
Ground Property and Equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 7,665	6,581
Estimated useful life	3-40 years
Ground Property and Equipment | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life	40 years
Ground Property and Equipment | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life	3 years
Flight and Ground Equipment Under Capital Lease
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 1,147	1,056
Less: accumulated depreciation and amortization	$ (668)	(757)
Estimated useful life	Shorter of lease term or estimated useful life
Advance Payments for Equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 1,160	$ 1,059