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Summary of Significant Accounting Policies (Details) | 12 Months Ended
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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.

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