Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of unrecognized tax benefits
Balance at January 1,	$ 16	$ 26	$ 14
Additions based on tax positions related to the current year	2	2	4
Additions for tax positions of prior years	0	0	8
Reductions for tax positions of prior years	(1)	(8)	0
Reductions related to settlements with taxing authorities	0	(4)	0
Balance at December 31,	$ 17	$ 16	$ 26