Income Taxes (Unrecognized Tax Benefits) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unrecognized Tax Benefit Rollforward [Abstract]
Unrecognized Tax Benefits, Beginning Balance	$ 66	$ 65	$ 80
Additions for tax positions of prior years	83	6	1
Reductions for tax positions of prior years	(3)	(5)	0
Settlements	(20)	0	(7)
Statute of limitations	0	0	(9)
Unrecognized Tax Benefits, Ending Balance	126	66	65
Unrecognized Tax Benefits That Would Impact Effective Tax Rate	10
Increase in Unrecognized Tax Benefits is Reasonably Possible	83
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense [Abstract]
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense	2	1	$ 1
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued [Abstract]
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued	$ 25	$ 15