Financial Derivative Instruments - Offsetting of Derivative Liabilities (Details) - USD ($) $ in Millions		Sep. 30, 2017	Dec. 31, 2016
Fuel derivatives | Prepaid expenses and other current assets
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities		$ 64	$ 48
Gross asset amounts offset in the Balance Sheet		(64)	(48)
Liability derivative contracts, net		0	0
Fuel derivatives | Other Assets
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities		15	58
Gross asset amounts offset in the Balance Sheet		(15)	(58)
Liability derivative contracts, net	[1]	0	0
Fuel derivatives | Accrued Liabilities
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities		169	674
Gross asset amounts offset in the Balance Sheet		(89)	(516)
Liability derivative contracts, net	[1]	80	158
Interest rate derivatives | Other Noncurrent Liabilities
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities		21	35
Gross asset amounts offset in the Balance Sheet		0	0
Liability derivative contracts, net	[1]	$ 21	$ 35

[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 5.