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Derivative Instruments Derivatives and hedging (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Mar. 26, 2018
Feb. 28, 2018
Derivatives, Fair Value [Line Items]            
Weather hedges term     10 years      
Gains (Losses) in Revenue [Member] | Weather Hedge Swaps [Member]            
Derivatives, Fair Value [Line Items]            
Recognized gain (loss) on weather hedges [1] $ 0 $ 0 $ (4) $ 1    
2018 to 2019 [Member] | Natural Gas Distribution [Member]            
Derivatives, Fair Value [Line Items]            
Weather hedge bilateral cap amount     9      
2018 to 2019 [Member] | Electric Transmission and Distribution [Member]            
Derivatives, Fair Value [Line Items]            
Weather hedge bilateral cap amount     8      
2018 to 2019 [Member] | Gains (Losses) in Revenue [Member] | Weather Hedge Swaps [Member] | Electric Transmission and Distribution [Member]            
Derivatives, Fair Value [Line Items]            
Recognized gain (loss) on weather hedges 0 0 0 0    
2017 to 2018 [Member] | Natural Gas Distribution [Member]            
Derivatives, Fair Value [Line Items]            
Weather hedge bilateral cap amount     8      
2017 to 2018 [Member] | Electric Transmission and Distribution [Member]            
Derivatives, Fair Value [Line Items]            
Weather hedge bilateral cap amount     9      
2017 to 2018 [Member] | Gains (Losses) in Revenue [Member] | Weather Hedge Swaps [Member] | Electric Transmission and Distribution [Member]            
Derivatives, Fair Value [Line Items]            
Recognized gain (loss) on weather hedges 0 0 (4) 0    
2016 to 2017 [Member] | Electric Transmission and Distribution [Member]            
Derivatives, Fair Value [Line Items]            
Weather hedge bilateral cap amount     9      
2016 to 2017 [Member] | Gains (Losses) in Revenue [Member] | Weather Hedge Swaps [Member] | Electric Transmission and Distribution [Member]            
Derivatives, Fair Value [Line Items]            
Recognized gain (loss) on weather hedges 0 0 0 1    
Houston Electric [Member] | January thru February [Member] | Treasury Lock [Member]            
Derivatives, Fair Value [Line Items]            
Aggregate notional amount 200   200      
Effective portion of realized gains (losses)     5      
CERC Corp [Member] | Gains (Losses) in Revenue [Member] | Weather Hedge Swaps [Member]            
Derivatives, Fair Value [Line Items]            
Recognized gain (loss) on weather hedges [1] 0 0 0 0    
CERC Corp [Member] | 2018 to 2019 [Member] | Gains (Losses) in Revenue [Member] | Weather Hedge Swaps [Member] | Natural Gas Distribution [Member]            
Derivatives, Fair Value [Line Items]            
Recognized gain (loss) on weather hedges 0 0 0 0    
CERC Corp [Member] | 2017 to 2018 [Member] | Gains (Losses) in Revenue [Member] | Weather Hedge Swaps [Member] | Natural Gas Distribution [Member]            
Derivatives, Fair Value [Line Items]            
Recognized gain (loss) on weather hedges 0 $ 0 0 $ 0    
CERC Corp [Member] | March [Member] | Treasury Lock [Member]            
Derivatives, Fair Value [Line Items]            
Aggregate notional amount $ 450   450      
Effective portion of realized gains (losses)     $ (1)      
General Mortgage Bonds Due 2048 [Member] | Houston Electric [Member]            
Derivatives, Fair Value [Line Items]            
Principal amount of debt issued           $ 400
Senior Notes [Member] | CERC Corp [Member]            
Derivatives, Fair Value [Line Items]            
Principal amount of debt issued         $ 600  
[1] Weather hedge gains (losses) are recorded in Revenues in the Condensed Statements of Consolidated Income.