Financial Derivative Instruments - Offsetting of Derivative Liabilities (Details) - USD ($) $ in Millions		Jun. 30, 2015	Dec. 31, 2014
Other Noncurrent Liabilities
Offsetting Liabilities [Line Items]
Liability derivative contracts, net		$ 105	$ 622
Fuel derivatives | Accrued Liabilities
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities		956	1,432
Gross asset amounts offset in the Balance Sheet		(639)	(1,258)
Liability derivative contracts, net	[1]	317	174
Fuel derivatives | Other Noncurrent Liabilities
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities		698	916
Gross asset amounts offset in the Balance Sheet		(647)	(355)
Liability derivative contracts, net	[1]	51	561
Interest rate derivatives | Other Noncurrent Liabilities
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities		54	61
Gross asset amounts offset in the Balance Sheet		0	0
Liability derivative contracts, net	[1]	$ 54	$ 61

[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.