Financial Derivative Instruments - Offsetting of Derivative Assets (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Fuel derivatives | Prepaid expenses and other current assets
Offsetting Assets [Line Items]
Gross amounts of recognized assets	$ 131		$ 375
Gross liability amounts offset in the Balance Sheet	(101)		(327)
Asset derivative contracts, net	30	[1]	48	[1]
Fuel derivatives | Other Assets
Offsetting Assets [Line Items]
Gross amounts of recognized assets	47		588
Gross liability amounts offset in the Balance Sheet	(38)		(367)
Asset derivative contracts, net	9		221
Fuel derivatives | Accrued Liabilities
Offsetting Assets [Line Items]
Gross amounts of recognized assets	87		10
Gross liability amounts offset in the Balance Sheet	(87)		(10)
Asset derivative contracts, net	0		0
Fuel derivatives | Other Noncurrent Liabilities
Offsetting Assets [Line Items]
Gross amounts of recognized assets	149		0
Gross liability amounts offset in the Balance Sheet	(149)		0
Asset derivative contracts, net	0		0
Interest rate derivatives | Other Assets
Offsetting Assets [Line Items]
Gross amounts of recognized assets	22		31
Gross liability amounts offset in the Balance Sheet	0		0
Asset derivative contracts, net	$ 22		$ 31

[1] Offsetting of derivative assets (in millions) (i) (ii) (iii) = (i) + (ii) (i) (ii) (iii) = (i) + (ii) June 30, 2013 December 31, 2012 Description Balance Sheet location Gross amounts of recognized assets Gross amounts offset in the Balance Sheet Net amounts of assets presented in the Balance Sheet Gross amounts of recognized assets Gross amounts offset in the Balance Sheet Net amounts of assets presented in the Balance Sheet Fuel derivative contracts Prepaid expenses and other current assets $131 $(101) $30(a)$375 $(327) $48(a)Fuel derivative contracts Other assets $47 $(38) $9 $588 $(367) $221 Fuel derivative contracts Accrued liabilities $87 $(87) $— $10 $(10) $— Fuel derivative contracts Other noncurrent liabilities $149 $(149) $— $— $— $— Interest rate derivative contracts Other assets $22 $— $22 $31 $— $31 (a) Amounts included in Prepaid expenses and other current a