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Derivative Instruments Derivatives and Hedging (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Derivatives, Fair Value [Line Items]          
Weather hedges term     10 years    
Derivative Assets Fair Value $ 277   $ 277   $ 220
Derivative Liabilities Fair Value 1,057   1,057   833
Gains (Losses) in Revenue [Member] | Weather Hedge Swaps [Member]          
Derivatives, Fair Value [Line Items]          
Recognized gain (loss) on weather hedges [1] 0 $ 0 3 $ (4)  
2018 to 2019 [Member] | Natural Gas Distribution [Member]          
Derivatives, Fair Value [Line Items]          
Weather hedge bilateral cap amount 9   9    
2018 to 2019 [Member] | Houston Electric T&D [Member]          
Derivatives, Fair Value [Line Items]          
Weather hedge bilateral cap amount 8   8    
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    
2018 to 2019 [Member] | Gains (Losses) in Revenue [Member] | Weather Hedge Swaps [Member] | Houston Electric T&D [Member]          
Derivatives, Fair Value [Line Items]          
Recognized gain (loss) on weather hedges 0   3    
2017to2018 [Member] | Natural Gas Distribution [Member]          
Derivatives, Fair Value [Line Items]          
Weather hedge bilateral cap amount   8   8  
2017to2018 [Member] | Houston Electric T&D [Member]          
Derivatives, Fair Value [Line Items]          
Weather hedge bilateral cap amount   9   9  
2017to2018 [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  
2017to2018 [Member] | Gains (Losses) in Revenue [Member] | Weather Hedge Swaps [Member] | Houston Electric T&D [Member]          
Derivatives, Fair Value [Line Items]          
Recognized gain (loss) on weather hedges   0   (4)  
Houston Electric [Member]          
Derivatives, Fair Value [Line Items]          
Derivative Assets Fair Value 0   0   0
Derivative Liabilities Fair Value 0   0   24
CERC Corp [Member]          
Derivatives, Fair Value [Line Items]          
Derivative Assets Fair Value 277   277   220
Derivative Liabilities Fair Value 205   205   208
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    
CERC Corp [Member] | 2017to2018 [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  
Not Designated as Hedging Instrument, Economic Hedge [Member] | Interest Rate Contract [Member]          
Derivatives, Fair Value [Line Items]          
Aggregate notional amount 84 [2]   84 [2]   0
Not Designated as Hedging Instrument, Economic Hedge [Member] | Houston Electric [Member] | Interest Rate Contract [Member]          
Derivatives, Fair Value [Line Items]          
Aggregate notional amount 0   0   0
Cash Flow Hedging [Member] | Interest Rate Contract [Member]          
Derivatives, Fair Value [Line Items]          
Aggregate notional amount 0 [2]   0 [2]   450
Cash Flow Hedging [Member] | Houston Electric [Member] | Interest Rate Contract [Member]          
Derivatives, Fair Value [Line Items]          
Aggregate notional amount $ 0   $ 0   $ 450
[1]
Weather hedge gains (losses) are recorded in Revenues in the Condensed Statements of Consolidated Income.
[2]
Relates to interest rate derivative instruments at SIGECO.