Income Taxes (Changes in Liability for Unrecognized Tax Benefits) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning balance of gross unrecognized tax benefits	$ 22	$ 26	$ 77
Increases: Positions taken during a prior period	4	14	17
Decreases: Positions taken during a prior period	(3)	(14)	(9)
Decreases: Settlements with taxing authorities	(4)		(52)
Decreases: Reductions resulting from lapse of statute of limitations	(1)	(4)	(7)
Ending balance of gross unrecognized tax benefits	18	22	26
Unrecognized tax benefits that would favorably impact the effective tax rate	$ 17	$ 18	$ 20