Income Taxes (Summary of Unrecognized Tax Benefits) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits — January 1	$ 47	$ 44	$ 30
Current year tax positions	12	4	9
Prior year tax positions	(22)
Prior year tax positions		1	7
Acquisitions	30	0	0
Settlements	0	(2)	(2)
Unrecognized tax benefits — December 31	$ 67	$ 47	$ 44