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Income Taxes - Additional Information (Detail) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Taxes [Line Items]            
Federal statutory income tax rate       35.00%   35.00%
Deferred income tax expense       $ 945 $ 1,648 $ (3,177)
Income tax benefit       192    
Provisional one-time transition tax on previously deferred foreign earnings       19    
Undistributed cumulative earnings of foreign subsidiaries contributing to one-time transition tax $ 122     122    
Income tax adjustments (192) $ 180   (192) 180 (3,130)
Valuation allowance 63 68   63 68  
Unrecognized tax benefits 21 $ 74   21 74 24
Unrecognized tax benefits that would impact effective tax rate if recognized 21     $ 21    
Scenario Forecast            
Income Taxes [Line Items]            
Federal statutory income tax rate     21.00%      
Minimum            
Income Taxes [Line Items]            
State tax benefits of the NOL carry forwards expiration period, years       5 years    
Maximum            
Income Taxes [Line Items]            
State tax benefits of the NOL carry forwards expiration period, years       20 years    
2026            
Income Taxes [Line Items]            
Net operating loss expiration amount 200     $ 200    
2029            
Income Taxes [Line Items]            
Net operating loss expiration amount 500     500    
Thereafter            
Income Taxes [Line Items]            
Net operating loss expiration amount 1,700     1,700    
Fuel | Designated as Hedging Instrument            
Income Taxes [Line Items]            
Deferred income tax expense         $ 180 $ 180
Federal            
Income Taxes [Line Items]            
NOL carry forwards 2,400     2,400    
State            
Income Taxes [Line Items]            
NOL carry forwards 49     49    
NOL carry forwards, net of valuation allowance 52     52    
Valuation allowance $ 63     $ 63