Comprehensive Income - Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Fuel and oil	$ 1,003	$ 941	$ 2,915	$ 2,696
Interest Expense	(28)	(31)	(84)	(93)
Less: Tax Expense	(288)	$ (230)	(913)	$ (1,016)
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Net of Tax	96		288
Fuel derivatives | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Less: Tax Expense	57		166
Net of Tax	94		282
Interest rate derivatives | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Less: Tax Expense	0		2
Net of Tax	2		6
Fuel derivatives
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Reclassification to earnings	151		448
Interest rate derivatives
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Reclassification to earnings	$ 2		$ 8