Income Taxes (Net Deferred Tax Asset (Liability)) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Current deferred tax asset	$ 3	$ 16
Current deferred tax liability	(829)	(729)
Noncurrent deferred tax asset	1,272	1,439
Noncurrent deferred tax liability	(75)	(82)
Net deferred tax asset	$ 371	$ 644