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Summary of Significant Accounting Policies Regulatory Assets and Liabilities (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Regulatory Assets and Liabilities [Line Items]    
Current regulatory assets $ 66 $ 60
Non-current regulatory assets 245 288
Total regulatory assets(a) 311 348
Current regulatory liabilities 29 107
Non-current regulatory liabilities 206 236
Total regulatory liabilities(b) 235 $ 343
Regulatory assets recoverable without earning a return $ 98  
Regulatory assets, weighted average remaining recovery period 23 years  
Remaining Amounts of Regulatory Liabilities Subject to Crediting Period $ 136  
Remaining Recovery Period of Regulatory Liabilities Subject to Defined Crediting Period 18 years  
Remaining Amounts of Regulatory Liabilities Not Subject to Defined Crediting Period $ 70  
Loss on Disposal of Assets [Member]    
Regulatory Assets and Liabilities [Line Items]    
Total regulatory assets(a) 176  
Income Tax Gross Up on AFUDC Equity [Member]    
Regulatory Assets and Liabilities [Line Items]    
Total regulatory assets(a) 53  
Other Regulatory Assets (Liabilities) [Member]    
Regulatory Assets and Liabilities [Line Items]    
Total regulatory assets(a) $ 82