Income Taxes Income Taxes - Addtional Information (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability	$ (82)	$ (1,200)
Deferred Tax Assets, Net	710	797
Domestic Tax Authority [Member]
Operating Loss Carryforwards [Line Items]
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability	(569)	$ (442)
State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability	$ (42)