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Fair Value Measurements (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Cash flow hedge gain (loss)   $ (3) $ (5) $ (4)  
Derivative instruments $ 21   21   $ 8
Cash Flow Hedging | Designated as Hedging Instrument | Interest Rate Swap          
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Derivative notional amount 88   88    
Cash Flow Hedging | Other Liabilities | Designated as Hedging Instrument          
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Derivative instruments 10   10   5
EnerBank | Fair Value Hedging | Designated as Hedging Instrument | Interest Rate Swap Notes Receivable          
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Derivative notional amount 134   134    
EnerBank | Fair Value Hedging | Other Liabilities | Designated as Hedging Instrument          
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Derivative instruments 7   $ 7   $ 1
Maximum          
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Cash flow hedge gain (loss) $ (1)