Accumulated Other Comprehensive (Income) Loss - Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest Expense	$ 118	$ 131	$ 114
Fuel and oil	4,347	4,616	4,076
Less: Tax Expense	657	$ 699	$ (92)
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax	41
Fuel derivatives | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Fuel and oil	48
Less: Tax Expense	11
Net of tax	37
Interest rate derivatives | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest Expense	5
Less: Tax Expense	1
Net of tax	$ 4