Financial Derivative Instruments - Offsetting of Derivative Liabilities (Details) - USD ($) $ in Millions		Jun. 30, 2016	Dec. 31, 2015
Other Noncurrent Liabilities
Offsetting Liabilities [Line Items]
Liability derivative contracts, net		$ 17	$ 74
Fuel derivatives | Prepaid expenses and other current assets
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities		19	26
Gross asset amounts offset in the Balance Sheet		(19)	(26)
Liability derivative contracts, net	[1]	0	0
Fuel derivatives | Other Assets
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities		11	0
Gross asset amounts offset in the Balance Sheet		(11)	0
Liability derivative contracts, net	[1]	0	0
Fuel derivatives | Accrued Liabilities
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities		1,222	2,380
Gross asset amounts offset in the Balance Sheet		(957)	(1,737)
Liability derivative contracts, net	[1]	265	643
Fuel derivatives | Other Noncurrent Liabilities
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities		494	1,010
Gross asset amounts offset in the Balance Sheet		(494)	(985)
Liability derivative contracts, net	[1]	0	25
Interest rate derivatives | Other Noncurrent Liabilities
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities		44	49
Gross asset amounts offset in the Balance Sheet		0	0
Liability derivative contracts, net	[1]	$ 44	$ 49

[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.