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Accumulated Other Comprehensive Income (Loss) - Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]      
Interest Expense $ 114 $ 122 $ 121
Fuel and oil 3,940 3,647 3,616
Less: Tax Expense (237) $ 1,303 $ 1,298
Reclassification out of Accumulated Other Comprehensive Income      
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]      
Net of tax 356    
Fuel derivatives | Reclassification out of Accumulated Other Comprehensive Income      
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]      
Fuel and oil 552    
Less: Tax Expense 203    
Net of tax 349    
Interest rate derivatives | Reclassification out of Accumulated Other Comprehensive Income      
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]      
Interest Expense 11    
Less: Tax Expense 4    
Net of tax $ 7