Income Taxes Components of Deferred Tax Assets / (Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Components of Deferred Tax Assets and Liabilities [Line Items]
Deferred Tax Liabilities, Property and Equipment	$ (13,860)	$ (13,583)
Deferred Tax Liabilities, Other	(346)	(391)
Total Deferred Tax Liabilities	(14,206)	(13,974)
Deferred Tax Assets, Casualty and environmental	122	159
Deferred Tax Assets, Compensation and Benefits	165	112
Deferred Tax Assets, Long-term Debt Fair Value Adjustment Under Acquisition Method Accounting	27	32
Deferred Tax Asset, Intangible Assets and Liabilities	16	32
Deferred Tax Asset, Pension and Retiree Health and Welfare Benefits	16	16
Deferred Tax Assets, Other	65	171
Total Deferred Tax Assets	411	522
Net Deferred Tax Liability	$ (13,795)	$ (13,452)