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Summary of Significant Accounting Policies (Details)
 12 Months Ended
Dec. 31, 2017
USD ($)
aircraft
$ / shares
Dec. 31, 2016
USD ($)
$ / shares
Dec. 31, 2015
USD ($)
$ / shares
Accounting Policies [Abstract]      
Inventory reserve for obsolescence $ 45,000,000 $ 57,000,000  
Frequent Flier Liability, Current 1,600,000,000 1,400,000,000  
Capitalized computer software, net 654,000,000 544,000,000  
Computer software depreciation expense 168,000,000 111,000,000 $ 106,000,000
Capitalized computer software, Impairments 0 0 0
Advertising costs $ 224,000,000 232,000,000 218,000,000
Concentration Risk [Line Items]      
Percentage Of Employees Represented By Unions Under Collective Bargaining Agreements 4.90%    
Change in Accounting Estimate [Line Items]      
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Operating Results $ 544,000,000 544,000,000 427,000,000
Change in Accounting Estimate Financial Effect on Depreciation Expense, Depreciation Estimate 21,000,000 123,000,000  
Special revenue adjustment 0 0 172,000,000
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Net Income $ 496,000,000 $ 293,000,000 $ 227,000,000
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Basic Earnings Per Share | $ / shares $ 0.82 $ 0.47 $ 0.34
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Diluted Earnings Per Share | $ / shares $ 0.82 $ 0.46 $ 0.34
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 $ 513,000,000  
Weighted-average useful life (in years) 14 years    
Accumulated amortization $ 89,000,000 87,000,000  
Amortization of intangible assets 13,000,000 17,000,000 $ 19,000,000
Impairment of Intangible Assets, Indefinite-lived (Excluding Goodwill) 0 21,000,000 $ 0
Finite-Lived Intangible Assets, Future Amortization Expense [Abstract]      
2017 13,000,000    
2018 13,000,000    
2019 12,000,000    
2020 12,000,000    
2021 12,000,000    
Thereafter 56,000,000    
Fuel derivatives      
Accounting Policies [Abstract]      
Total collateral already posted aggregate fair value 15,000,000 (301,000,000)  
Change in Accounting Estimate [Line Items]      
Collateral Already Posted, Aggregate Fair Value 15,000,000    
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    
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 10 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 38,000,000  
Weighted-average useful life (in years) 10 years    
Accumulated amortization $ 23,000,000 32,000,000  
Owned domestic slots      
Acquired Finite-Lived Intangible Assets [Line Items]      
Gross carrying amount of indefinite-lived intangible assets $ 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 (b)      
Acquired Finite-Lived Intangible Assets [Line Items]      
Gross carrying amount of finite-lived intangible assets $ 180,000,000 [2] 180,000,000  
Weighted-average useful life (in years) 15 years    
Accumulated amortization [2] $ 66,000,000 $ 55,000,000  
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
[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.