Income Taxes Components of Deferred Tax Assets / (Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Components of Deferred Tax Assets and Liabilities [Line Items]
Deferred Tax Liabilities, Property and Equipment	$ (19,660)	$ (18,624)
Deferred Tax Liabilities, Other	(341)	(363)
Deferred Tax Liabilities, Gross	(20,001)	(18,987)
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits	328	335
Deferred Tax Assets, Tax Deferred Expense, Casualty and environmental	293	307
Deferred Tax Asset, Tax Deferred Expense, Intangible Assets and Liabilities	100	132
Deferred Tax Assets, Tax Deferred Expense, Long-term Debt Fair Value Adjustment	96	117
Deferred Tax Asset, Deferred Tax Expense, Pension and Retiree Health and Welfare Benefits	126	202
Deferred Tax Assets, Other	231	248
Deferred Tax Assets, Gross	1,174	1,341
Deferred Tax Liabilities, Net	(18,827)	(17,646)
Deferred Tax Liabilities, Net, Noncurrent	(19,072)	(18,001)
Deferred Tax Assets, Net, Current	$ 245	$ 355