Income Taxes (Reconciliation of Beginning and Ending Unrecognized Tax Benefits) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of Unrecognized Tax Benefits
Balance at beginning of year	$ 158	$ 258	$ 227
Additions based on tax positions related to the current year	15	15	15
Additions for tax positions of prior years	29	57	29
Reductions for tax positions due to lapse of statutes of limitations	(4)	(4)	(2)
Reductions for tax positions of prior years	(28)	(86)	0
Settlements	(1)	(82)	(11)
Balance at end of year	$ 169	$ 158	$ 258