Income Taxes (details) - Additional Tax Details - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits, balance at January 1	$ 13	$ 16
Additions for tax positions of prior years		3
Reductions for tax positions of prior years	(1)	(6)
Reductions based on tax positions related to current year	(6)
Unrecognized tax benefits, balance at December 31	6	13	$ 16
Unrecognized tax benefits that, if recognized, would affect the annual effective tax rate	3	7
Amount of unrecognized tax benefits that are tax positions for which the ultimate deductibility is certain, but for which there is uncertainty about the timing of deductibility	3	6
Amount of interest recognized on unrecognized tax benefits in income taxes		31
Amount of interest benefit recognized on unrecognized tax benefits in income taxes	(33)		$ (32)
Accrued payment of interest balance for unrecognized tax benefits	$ 25	$ 57