Summary of Significant Accounting Policies (Details)		12 Months Ended
		Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)		Dec. 31, 2017 USD ($) aircraft $ / shares
Accounting Policies [Abstract]
Capitalized computer software, net		$ 630,000,000	$ 674,000,000
Computer software depreciation expense		177,000,000	155,000,000		$ 168,000,000
Capitalized computer software, Impairments		0
Advertising costs		$ 212,000,000	215,000,000		224,000,000
Concentration Risk [Line Items]
Percentage Of Employees Represented By Unions Under Collective Bargaining Agreements		40.00%
Number of Aircraft Grounded Under Emergency Order		34
Change in Accounting Estimate [Line Items]
Change in Accounting Estimate Financial Effect on Depreciation Expense, Depreciation Estimate					21,000,000
Change in Accounting Estimate, Financial Effect on Net Income, Depreciation Adjustment	[1]				$ (19,000,000)
Change in Accounting Estimate, Financial Effect on Net Income Per Share, Depreciation Estimate | $ / shares					$ (0.03)
ChangeinAccountingEstimateFinancialEffectonNetIncomePerShareDilutedDepreciationEstimate | $ / shares					$ (0.03)
Remaining -300 aircraft grounded in September 2017 | aircraft					61
Acquired Finite-Lived Intangible Assets [Line Items]
Gross carrying amount		$ 309,000,000	502,000,000
Weighted-average useful life (in years)		12 years
Accumulated amortization		$ 13,000,000	103,000,000
Amortization of intangible assets		15,000,000	16,000,000		$ 13,000,000
Fuel derivatives
Accounting Policies [Abstract]
Total collateral already posted aggregate fair value		25,000,000	0
Change in Accounting Estimate [Line Items]
Collateral Already Posted, Aggregate Fair Value		25,000,000
Other Noncurrent Liabilities | Interest rate derivatives
Change in Accounting Estimate [Line Items]
Collateral Already Posted, Aggregate Fair Value		$ 0	0
Flight equipment
Property, Plant and Equipment [Line Items]
Maximum percentage of cost estimated as residual value (in hundredths)		15.00%
Flight equipment | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant, and Equipment Useful Life		25 years
Ground property and equipment
Property, Plant and Equipment [Line Items]
Minimum percentage of cost estimated residual value (in hundredths)		0.00%
Maximum percentage of cost estimated as residual value (in hundredths)		10.00%
Ground property and equipment | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant, and Equipment Useful Life		5 years
Ground property and equipment | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant, and Equipment Useful Life		30 years
Software | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant, and Equipment Useful Life		5 years
Software | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant, and Equipment Useful Life		15 years
Customer relationships/marketing agreements
Acquired Finite-Lived Intangible Assets [Line Items]
Gross carrying amount of finite-lived intangible assets		$ 14,000,000	27,000,000
Weighted-average useful life (in years)		12 years
Accumulated amortization		$ 13,000,000	25,000,000
Owned domestic slots (a)
Acquired Finite-Lived Intangible Assets [Line Items]
Gross carrying amount of indefinite-lived intangible assets	[2]	295,000,000	295,000,000
Gate leasehold rights (b)
Acquired Finite-Lived Intangible Assets [Line Items]
Gross carrying amount of finite-lived intangible assets		$ 0	180,000,000	[3]
Weighted-average useful life (in years)		0 years
Accumulated amortization		$ 0	$ 78,000,000	[3]
Unionized Employees concentration risk
Concentration Risk [Line Items]
The percentage of Company's employees that are unionized and covered by collective bargaining agreements (in hundredths)		83.00%

[1] net of profitsharing benefit and income taxes

[2] Intangible assets primarily consist of acquired rights to certain airport owned takeoff and landing slots (a "slot" is the right of an air carrier, pursuant to regulations of the Federal Aviation Administration ("FAA"), to operate a takeoff or landing at a specific time at certain airports) at certain domestic slot-controlled airports, and certain intangible assets acquired.

[3] Airport gate leasehold rights are classified as right-of-use assets upon adoption of the New Lease Standard. See Note 7.