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Risk Management (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Jan. 31, 2019
Dec. 31, 2018
Derivative Liability, Collateral, Right to Reclaim Cash, Offset           $ 11
Net gain or (loss) recognized in the statement of income offset by regulatory accounting $ 24 $ 8 $ 5 $ 2    
Derivative, Net Liability Position, Aggregate Fair Value 118   118      
Collateral Already Posted, Aggregate Fair Value 21   21      
Collateral cash requirement 109   109      
Deposit Assets 0   0      
Derivative Liability 0   0   $ 5 4
Natural Gas [Member]            
Derivative Instruments and Hedges, Liabilities 2   2     4
4911 Electric Services [Member]            
Derivative Instruments and Hedges, Liabilities 0   0     84
Liabilities, Total [Member]            
Derivative Instruments and Hedges, Liabilities $ 2   $ 2     88
Fair Value, Inputs, Level 2 [Member]            
Derivative Liability           $ 4