Summary of Significant Accounting Policies (Details)		12 Months Ended
		Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) aircraft $ / shares	Dec. 31, 2016 USD ($) $ / shares
Accounting Policies [Abstract]
Inventory reserve for obsolescence		$ 2,000,000	$ 45,000,000
Capitalized computer software, net		674,000,000	654,000,000
Computer software depreciation expense		155,000,000	168,000,000	$ 111,000,000
Capitalized computer software, Impairments		0	0	0
Advertising costs		$ 215,000,000	224,000,000	232,000,000
Concentration Risk [Line Items]
Percentage Of Employees Represented By Unions Under Collective Bargaining Agreements		43.00%
Change in Accounting Estimate [Line Items]
Change in Accounting Estimate Financial Effect on Depreciation Expense, Depreciation Estimate			21,000,000	123,000,000
Change in Accounting Estimate, Financial Effect on Net Income, Depreciation Adjustment	[1]		$ (19,000,000)	$ (66,000,000)
Change in Accounting Estimate, Financial Effect on Net Income Per Share, Depreciation Estimate | $ / shares			$ (0.03)	$ (0.11)
ChangeinAccountingEstimateFinancialEffectonNetIncomePerShareDilutedDepreciationEstimate | $ / shares			$ (0.03)	$ (0.10)
Remaining -300 aircraft grounded in September 2017 | aircraft			61
Acquired Finite-Lived Intangible Assets [Line Items]
Gross carrying amount		$ 502,000,000	$ 502,000,000
Weighted-average useful life (in years)		14 years
Accumulated amortization		$ 103,000,000	89,000,000
Amortization of intangible assets		16,000,000	13,000,000	$ 17,000,000
Loss on asset impairment		0	0	$ 21,000,000
Finite-Lived Intangible Assets, Future Amortization Expense [Abstract]
2019		13,000,000
2020		12,000,000
2021		12,000,000
2022		12,000,000
2023		12,000,000
Thereafter		45,000,000
Fuel derivatives
Accounting Policies [Abstract]
Total collateral already posted aggregate fair value		0	(15,000,000)
Change in Accounting Estimate [Line Items]
Collateral Already Posted, Aggregate Fair Value		0
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
Property, Plant and Equipment [Member] | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant, and Equipment Useful Life		5 years
Property, Plant and Equipment [Member] | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant, and Equipment Useful Life		30 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%
AssetsConstructedForOthers
Property, Plant and Equipment [Line Items]
Minimum percentage of cost estimated residual value (in hundredths)		17.00%
Maximum percentage of cost estimated as residual value (in hundredths)		75.00%
AssetsConstructedForOthers | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant, and Equipment Useful Life		5 years
AssetsConstructedForOthers | 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		$ 27,000,000	27,000,000
Weighted-average useful life (in years)		10 years
Accumulated amortization		$ 25,000,000	23,000,000
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
Leased domestic slots (a)
Acquired Finite-Lived Intangible Assets [Line Items]
Weighted-average useful life (in years)	[2]	0 years
Gate leasehold rights (a)
Acquired Finite-Lived Intangible Assets [Line Items]
Gross carrying amount of finite-lived intangible assets	[2]	$ 180,000,000	180,000,000
Weighted-average useful life (in years)	[2]	15 years
Accumulated amortization	[2]	$ 78,000,000	$ 66,000,000
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 leasehold rights to certain airport owned gates, takeoff and landing slots (a "slot" is the right of an air carrier, pursuant to regulations of the 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.