Income Taxes - Deferred Income Tax Valuation Allowance Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred Tax Assets Valuation Allowance [Roll Forward]
Beginning balance of deferred income tax valuation allowance	$ 122	$ 72	$ 75
Increases charged to income tax expense	0	28	1
Charges against goodwill	15	22	1
Decreases	(11)	0	(5)
Ending balance of deferred income tax valuation allowance	$ 126	$ 122	$ 72