Fair Value Measurements (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash flow hedge gain (loss)	$ 1	$ 0	$ 6	$ 4
Derivative instruments	20		20		$ 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	9		9		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