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Income Taxes Income taxes (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Operating Loss Carryforwards [Line Items]    
Deferred Tax Liabilities, Regulatory Assets and Liabilities $ 340  
Increase in net deferred tax regulatory liabilitily 357  
Tax Credit Carryforward, Federal 50  
Deferred Tax Assets, Valuation Allowance 0 $ 0
Unrecognized Tax Benefits 0 $ 0
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability $ 17