Summary of Significant Accounting Policies - Property and Equipment, net (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Less: accumulated depreciation and amortization	$ (17,511)	$ (17,027)
Total property and equipment, net	26,529	31,310
Flight equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 31,572	36,713
Flight equipment | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life	20 years
Flight equipment | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life	34 years
Ground property and equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 6,387	5,721
Ground property and equipment | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life	3 years
Ground property and equipment | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life	40 years
Information technology-related assets
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 3,403	3,276
Information technology-related assets | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life	3 years
Information technology-related assets | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life	15 years
Flight and ground equipment under capital lease
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 1,795	1,608
Less: accumulated depreciation and amortization	(793)	(546)
Advance payments for equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 883	$ 1,019