Income Taxes Income Taxes - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Tax Contingency [Line Items]
Tax credit carryforward	$ 3		$ 3
Unrecognized tax benefit, decrease from state income tax expense			2
Unrecognized tax benefits, interest expense			4
Income tax expense (benefit)	$ 1	$ 0	(8)	$ 2
State and Local Jurisdiction [Member]
Income Tax Contingency [Line Items]
Unrecognized tax benefits, decrease resulting from settlements with taxing authorities			$ 13