Financial Derivative Instruments - Offsetting of Derivative Assets (Details) - USD ($) $ in Millions		Sep. 30, 2017	Dec. 31, 2016
Offsetting Assets [Line Items]
Asset derivative contracts, net		$ 84	$ 120
Fuel derivatives | Prepaid expenses and other current assets
Offsetting Assets [Line Items]
Gross amounts of recognized assets		64	61
Gross liability amounts offset in the Balance Sheet		(64)	(48)
Asset derivative contracts, net		0	13
Fuel derivatives | Other Assets
Offsetting Assets [Line Items]
Gross amounts of recognized assets		99	178
Gross liability amounts offset in the Balance Sheet		(15)	(58)
Asset derivative contracts, net	[1]	84	120
Fuel derivatives | Accrued Liabilities
Offsetting Assets [Line Items]
Gross amounts of recognized assets		89	516
Gross liability amounts offset in the Balance Sheet		(89)	(516)
Asset derivative contracts, net	[1]	$ 0	$ 0

[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.