Income Taxes - Deferred Taxes (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Deferred tax assets:
Net operating loss carryforwards	$ 2,485	$ 3,838
Pension, postretirement and other benefits	5,259	5,444
Fuel derivatives MTM adjustments	112	282
Alternative minimum tax credit carryforward	379	379
Deferred revenue	1,544	1,522
Other	963	1,047
Valuation allowance	(40)	(56)	$ (46)	$ (177)
Total deferred tax assets	10,702	12,456
Deferred tax liabilities:
Depreciation	5,701	5,490
Intangible assets	1,691	1,681
Other	246	329
Total deferred tax liabilities	7,638	7,500
Net deferred tax assets	$ 3,064	$ 4,956