Income Taxes (Reconciliation of Beginning and Ending Unrecognized Tax Benefits) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits
Balance at beginning of year	$ 169	$ 158	$ 258
Additions based on tax positions related to the current year	0	15	15
Additions for tax positions of prior years	129	29	57
Reductions for tax positions due to lapse of statutes of limitations	(4)	(4)	(4)
Reductions for tax positions of prior years	(208)	(28)	(86)
Settlements	(20)	(1)	(82)
Balance at end of year	$ 66	$ 169	$ 158