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Summary of Significant Accounting Policies (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Property, Plant and Equipment [Line Items]      
Other Investments $ 30 $ 1  
Business days after the invoice date 16    
Days after the due date 45    
Wholesale accounts receivable written off $ 0 0 $ 0
Margin deposit 11 8  
Letters of Credit Outstanding, Amount $ 31 $ 17  
Public Utilities, Allowance for Funds Used During Construction, Rate 7.30% 7.30% 7.30%
Public Utilities, Allowance for Funds Used During Construction, Capitalized Interest $ 6 $ 11 $ 13
Allowance for equity funds used during construction $ 12 $ 21 $ 21
Depreciation expense rate 3.60% 3.50% 3.60%
Finite-Lived Intangible Assets, Accumulated Amortization $ 296 $ 257  
Amortization of Intangible Assets 46 $ 44 $ 38
Future Amortization Expense, Year One 49    
Future Amortization Expense, Year Two 48    
Future Amortization Expense, Year Three 43    
Future Amortization Expense, Year Four 35    
Future Amortization Expense, Year Five 28    
Power Cost Deadband - Lower Threshold 15    
Power Cost Deadband - Upper Threshold $ 30    
Public Utilities, Approved Return on Equity, Percentage 9.60% 9.60% 9.68%
Regulatory Liability, Noncurrent $ 1,288 $ 958  
Franchise taxes 43 43 $ 43
Regulatory Assets, Noncurrent 438 498  
Defined Benefit Plan, Interest Cost 3    
Deferred Income Tax Charge [Member]      
Property, Plant and Equipment [Line Items]      
Regulatory Assets, Noncurrent 0 86  
Asset Retirement Obligation Costs [Member]      
Property, Plant and Equipment [Line Items]      
Regulatory Liability, Noncurrent 52 49  
Deferred Income Tax Charge [Member]      
Property, Plant and Equipment [Line Items]      
Regulatory Liability, Noncurrent $ 277 $ 0