Income Taxes - Deferred Taxes (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Components of Deferred Tax Assets [Abstract]
Net operating loss carryforwards	$ 4,782	$ 6,024
Pension, postretirement and other benefits	6,033	4,982
Fuel derivatives MTM adjustments	777	0
AMT credit carryforward	357	378
Deferred revenue	1,824	1,965
Other	659	698
Valuation allowance	(46)	(177)	(10,963)	(10,705)
Total deferred tax assets	14,386	13,870
Components of Deferred Tax Liabilities [Abstract]
Depreciation	4,663	4,799
Intangible assets	1,684	1,704
Other	444	639
Total deferred tax liabilities	6,791	7,142
Income Tax Disclosure [Abstract]
Deferred income taxes, net	3,275	1,736
Noncurrent deferred tax assets, net	4,320	4,992
Total deferred tax assets, net	$ 7,595	$ 6,728