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Differences between Net Income and Comprehensive Income (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Condensed Statement of Income Captions [Line Items]    
Net income $ 463 $ 339
Other Comprehensive Income (Loss), Net of Tax 84 11
Comprehensive Income (Loss), Net of Tax, Attributable to Parent 547 350
Fuel derivatives    
Condensed Statement of Income Captions [Line Items]    
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax 81 9
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Tax 25 5
Interest rate derivatives    
Condensed Statement of Income Captions [Line Items]    
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax 3 2
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Tax $ 0 $ 1
Reclassification out of Accumulated Other Comprehensive Income    
Condensed Statement of Income Captions [Line Items]    
Reclassification of Stranded amounts in AOCI to Retained Earnings (ASU 2018-02) 2