Income Taxes (Components of deferred tax assets and liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Reserves and allowances	$ 87	$ 103
Debt cancellation and other	13	33
Net operating loss and credit carryforwards	192	28
Total deferred tax assets	292	164
Property and equipment	(1,498)	(1,820)
Intangibles	(174)	(231)
Valuation allowance	(39)	(9)
Total deferred tax liability	(1,711)	(2,060)
Total deferred income tax liability	$ (1,419)	$ (1,896)