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Summary of Significant Accounting Policies Regulatory Assets and Liabilities (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Regulatory Assets and Liabilities [Line Items]    
Current regulatory assets $ 60 $ 49
Non-current regulatory assets 288 330
Total regulatory assets(a) 348 379
Current regulatory liabilities 107 101
Non-current regulatory liabilities 236 108
Total regulatory liabilities(b) 343 $ 209
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided $ 124  
Remaining Recovery Period of Regulatory Assets for which No Return on Investment During Recovery Period is Provided 17 years  
Remaining Amounts of Regulatory Non-current Liabilities Subject to Crediting Period $ 20  
Remaining Recovery Period of Regulatory Liabilities Subject to Defined Crediting Period 28 years  
Remaining Amounts of Regulatory Non-current Liabilities Not Subject to Defined Crediting Period $ 216  
Loss on Disposal of Assets [Member]    
Regulatory Assets and Liabilities [Line Items]    
Total regulatory assets(a) 193  
Income Tax Gross Up on AFUDC Equity [Member]    
Regulatory Assets and Liabilities [Line Items]    
Total regulatory assets(a) 55  
Other Regulatory Assets (Liabilities) [Member]    
Regulatory Assets and Liabilities [Line Items]    
Total regulatory assets(a) $ 100