Summary of Significant Accounting Policies - Narrative (Details) t in Millions	12 Months Ended
	Dec. 31, 2021 USD ($) t	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Summary of Significant Accounting Policies [Line Items]
Regional carrier expense	$ 1,736,000,000	$ 1,584,000,000	$ 2,158,000,000
Derivative contracts, net	17,000,000	1,000,000
Depreciation and amortization expense related to property and equipment	1,998,000,000	2,312,000,000	2,581,000,000
Amortization of capitalized software	301,000,000	304,000,000	239,000,000
Net book value of capitalized software	876,000,000	1,000,000,000
Restructuring charges	(19,000,000)	8,219,000,000	0
Fuel card obligation	1,100,000,000	1,100,000,000
Advertising expense	198,000,000	119,000,000	288,000,000
Expense related to carbon offset credits	95,000,000
Expense related to carbon offset credits, emissions in prior year	$ 30,000,000
Carbon emissions generated in prior year | t	13
Expense related to carbon offset credits, emissions in current year	$ 65,000,000
Effect of allocation
Summary of Significant Accounting Policies [Line Items]
Regional carrier expense		$ (900,000,000)	$ (1,400,000,000)
Fuel Card Obligation
Summary of Significant Accounting Policies [Line Items]
Purchasing card maximum limit	$ 1,100,000,000
Minimum
Summary of Significant Accounting Policies [Line Items]
Estimated residual value (percent)	5.00%
Maximum
Summary of Significant Accounting Policies [Line Items]
Estimated residual value (percent)	10.00%
Software and software development costs | Minimum
Summary of Significant Accounting Policies [Line Items]
Estimated useful life	3 years
Software and software development costs | Maximum
Summary of Significant Accounting Policies [Line Items]
Estimated useful life	10 years