Financial Derivative Instruments - Offsetting of Derivative Liabilities (Details) - USD ($) $ in Millions		Sep. 30, 2018	Dec. 31, 2017
Fuel derivatives | Prepaid expenses and other current assets
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities		$ 148	$ 50
Gross asset amounts offset in the Balance Sheet		(148)	(50)
Liability derivative contracts, net		0	0
Fuel derivatives | Other Assets
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities		17	0
Gross asset amounts offset in the Balance Sheet		(17)	0
Liability derivative contracts, net	[1]	0	0
Interest rate derivatives | Accrued Liabilities [Member]
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities		0	1
Gross asset amounts offset in the Balance Sheet		0	0
Liability derivative contracts, net		0	1
Interest rate derivatives | Other Noncurrent Liabilities
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities		22	21
Gross asset amounts offset in the Balance Sheet		0	0
Liability derivative contracts, net	[1]	$ 22	$ 21

[1] The net amounts of derivative assets and liabilities are reconciled to the individual line item amounts presented in the unaudited Condensed Consolidated Balance Sheet in Note 9.