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Derivative Instruments Derivatives and Hedging (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Weather hedges term 10 years    
Gains (Losses) in Revenue [Member] | Weather Hedge Swaps [Member]      
Recognized gain (loss) on weather hedges [1] $ (4) $ (1) $ 1
2018 to 2019 [Member] | Natural Gas Distribution [Member]      
Weather hedge bilateral cap amount 9    
2018 to 2019 [Member] | Electric Transmission & Distribution [Member]      
Weather hedge bilateral cap amount 8    
2018 to 2019 [Member] | Gains (Losses) in Revenue [Member] | Weather Hedge Swaps [Member] | Natural Gas Distribution [Member]      
Recognized gain (loss) on weather hedges 0 0 0
2018 to 2019 [Member] | Gains (Losses) in Revenue [Member] | Weather Hedge Swaps [Member] | Electric Transmission & Distribution [Member]      
Recognized gain (loss) on weather hedges 0 0 0
2017 to 2018 [Member] | Natural Gas Distribution [Member]      
Weather hedge bilateral cap amount 8    
2017 to 2018 [Member] | Electric Transmission & Distribution [Member]      
Weather hedge bilateral cap amount 9    
2017 to 2018 [Member] | Gains (Losses) in Revenue [Member] | Weather Hedge Swaps [Member] | Natural Gas Distribution [Member]      
Recognized gain (loss) on weather hedges (2) 0 0
2017 to 2018 [Member] | Gains (Losses) in Revenue [Member] | Weather Hedge Swaps [Member] | Electric Transmission & Distribution [Member]      
Recognized gain (loss) on weather hedges (2) 0 0
2016 to 2017 [Member] | Electric Transmission & Distribution [Member]      
Weather hedge bilateral cap amount 9    
2016 to 2017 [Member] | Gains (Losses) in Revenue [Member] | Weather Hedge Swaps [Member] | Electric Transmission & Distribution [Member]      
Recognized gain (loss) on weather hedges $ 0 (1) 1
CERC Corp [Member]      
Weather hedges term 10 years    
CERC Corp [Member] | Gains (Losses) in Revenue [Member] | Weather Hedge Swaps [Member]      
Recognized gain (loss) on weather hedges [1] $ (2) 0 0
CERC Corp [Member] | 2018 to 2019 [Member] | Natural Gas Distribution [Member]      
Weather hedge bilateral cap amount 9    
CERC Corp [Member] | 2018 to 2019 [Member] | Gains (Losses) in Revenue [Member] | Weather Hedge Swaps [Member] | Natural Gas Distribution [Member]      
Recognized gain (loss) on weather hedges 0 0 0
CERC Corp [Member] | 2017 to 2018 [Member] | Natural Gas Distribution [Member]      
Weather hedge bilateral cap amount 8    
CERC Corp [Member] | 2017 to 2018 [Member] | Gains (Losses) in Revenue [Member] | Weather Hedge Swaps [Member] | Natural Gas Distribution [Member]      
Recognized gain (loss) on weather hedges (2) 0 $ 0
Cash Flow Hedging [Member] | Interest Rate Contract [Member]      
Aggregate notional amount 450 0  
Cash Flow Hedging [Member] | CERC Corp [Member] | Interest Rate Contract [Member]      
Aggregate notional amount $ 450 $ 0  
[1] Weather hedge gains (losses) are recorded in Revenues in the Statements of Consolidated Income.