Income Taxes Components of Deferred Tax Assets / (Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Components of Deferred Tax Assets and Liabilities [Line Items]
Deferred Tax Liabilities, Property and Equipment	$ (13,583)	$ (20,634)
Deferred Tax Liabilities, Other	(391)	(340)
Total Deferred Tax Liabilities	(13,974)	(20,974)
Deferred Tax Assets, Casualty and environmental	159	278
Deferred Tax Assets, Compensation and Benefits	112	334
Deferred Tax Assets, Long-term Debt Fair Value Adjustment Under Acquisition Method Accounting	32	76
Deferred Tax Asset, Intangible Assets and Liabilities	32	72
Deferred Tax Asset, Pension and Retiree Health and Welfare Benefits	16	89
Deferred Tax Assets, Other	171	259
Total Deferred Tax Assets	522	1,108
Net Deferred Tax Liability	$ (13,452)	$ (19,866)