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Related Party Transactions and Major Customers (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
Sep. 30, 2012
Dec. 31, 2012
Related Party Transaction [Line Items]          
Investments in Money Pool $ 64   $ 64   $ 433
Notes Receivable from Parent 750   750   750
Interest Income (Expense) Net, Related Party 5 5 14 14  
Corporate service charges 36 33 109 105  
Sales to major customers 745 748 1,933 1,955  
Affiliates of NRG Energy, Inc. [Member]
         
Related Party Transaction [Line Items]          
Sales to major customers 202 196 494 488  
Affiliates of Energy Future Holdings Corp. [Member]
         
Related Party Transaction [Line Items]          
Sales to major customers 52 49 125 123  
August 1st, 2013 [Member] | CenterPoint Houston [Member] | Intercompany note payable
         
Related Party Transaction [Line Items]          
Extinguishment of Debt, Amount 92        
Debt Instrument, Interest Rate, Stated Percentage 4.00%   4.00%    
October 15th, 2013 [Member] | CenterPoint Houston [Member] | Intercompany note payable
         
Related Party Transaction [Line Items]          
Extinguishment of Debt, Amount $ 59        
Debt Instrument, Interest Rate, Stated Percentage 4.00%   4.00%