Income Taxes (Schedule of Unrecognized Tax Benefits and Liability for Contingent Income Taxes, Interest and Penalties) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Unrecognized tax benefits	$ 145	$ 79	$ 88	$ 123
Accrued interest and penalties	23	15	28
Tax credits and other indirect benefits	(35)	(31)	(40)
Liability for tax contingencies	$ 133	$ 63	$ 76