Income Taxes (Components of deferred tax assets and liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Components of Deferred Tax Assets [Abstract]
Reserves and allowances	$ 103	$ 112
Debt cancellation and other	33	48
Net operating loss and credit carryforwards	28	73
Total deferred tax assets	164	233
Components of Deferred Tax Liabilities [Abstract]
Property and equipment	(1,820)	(1,714)
Intangibles	(231)	(272)
Valuation allowance	(9)	(12)
Total deferred tax liability	(2,060)	(1,998)
Total deferred income tax (liability)	$ (1,896)	$ (1,765)