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Summary of Significant Accounting Policies - Regulatory Assets and Liabilities (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Regulatory Assets and Liabilities [Line Items]    
Current regulatory assets $ 25 $ 55
Non-current regulatory assets 231 212
Total regulatory assets(a) 256 267
Current regulatory liabilities 26 26
Non-current regulatory liabilities 169 189
Total regulatory liabilities(b) 195 $ 215
Regulatory assets recoverable without earning a return $ 119  
Regulatory assets, weighted average remaining recovery period 14 years  
Remaining Amounts of Regulatory Liabilities Subject to Crediting Period $ 112  
Remaining Recovery Period of Regulatory Liabilities Subject to Defined Crediting Period 17 years  
Remaining Amounts of Regulatory Liabilities Not Subject to Defined Crediting Period $ 57  
Loss on Disposal of Assets [Member]    
Regulatory Assets and Liabilities [Line Items]    
Total regulatory assets(a) 131  
Income Tax Gross Up on AFUDC Equity [Member]    
Regulatory Assets and Liabilities [Line Items]    
Total regulatory assets(a) 49  
Other Regulatory Assets (Liabilities) [Member]    
Regulatory Assets and Liabilities [Line Items]    
Total regulatory assets(a) $ 76