Income Taxes Income Taxes - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Tax Contingency [Line Items]
Tax credit carryforward	$ 3		$ 3
Unrecognized tax benefit, decrease from state income tax expense	1
Unrecognized tax benefits, interest expense	4
Income tax expense (benefit)	(9)	$ 1	$ (9)	$ 2
State and Local Jurisdiction [Member]
Income Tax Contingency [Line Items]
Unrecognized tax benefits, decrease resulting from settlements with taxing authorities	$ 13