Income Taxes - Deferred Taxes (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carryforwards	$ 1,440	$ 2,485
Pension, postretirement and other benefits	2,545	5,259
Alternative minimum tax credit carryforward	379	379
Deferred revenue	1,024	1,544
Other	746	1,075
Valuation allowance	(19)	(40)
Total deferred tax assets	6,115	10,702
Deferred tax liabilities:
Depreciation	3,936	5,701
Intangible assets	1,070	1,691
Other	174	246
Total deferred tax liabilities	5,180	7,638
Net deferred tax assets	$ 935	$ 3,064