Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Contingency [Line Items]
Unrecognized tax benefits, balance	$ 36	$ 45	$ 62
Additions for tax positions related to current year	2	3	3
Additions for tax positions related to prior years	0	0	0
Reductions for tax positions related to prior years	0	(7)	(18)
Reductions for settlements with taxing authorities	0	(1)	(2)
Reductions for expiring statutes	(1)	(4)	0
Unrecognized tax benefits, balance	37	36	45
Unrecognized tax benefits that would affect effective rate	$ 8	$ 8	$ 9