Income Taxes (Summary of Activities Related to 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	$ 38	$ 33	$ 34
Increases related to tax positions taken during a prior period	4	1	5
Increases related to tax positions during the current period	7	8	6
Decreases related to settlements with taxing authorities	(2)	0	(2)
Decrease as a result of a lapse of the applicable statute of limitations	(3)	(2)	(9)
Decreases related to tax positions taken during a prior period	(3)	(2)	(1)
Ending balance	$ 41	$ 38	$ 33