Comprehensive Income - Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Fuel and oil	$ (990)	$ (903)	$ (1,912)	$ (1,755)
Interest Expense	(27)	(32)	(56)	(62)
Less: Tax Expense	424	$ 484	626	$ 787
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Net of Tax	102		192
Fuel derivatives | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Fuel and oil	158		297
Less: Tax Expense	58		109
Net of Tax	100		188
Interest rate derivatives | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Interest Expense	3		6
Less: Tax Expense	1		2
Net of Tax	$ 2		$ 4