Fair Value Measurements (Narrative) (Details) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gain (loss) on derivatives	$ 0	$ 4,000,000
Derivative instruments	11,000,000	11,000,000	$ 3,000,000
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	94,000,000	94,000,000
Cash Flow Hedging | Other Liabilities | Designated as Hedging Instrument
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments	6,000,000	6,000,000	$ 2,000,000
Fair Value Hedging | EnerBank | 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	61,000,000	61,000,000
Fair Value Hedging | Other Liabilities | EnerBank | Designated as Hedging Instrument
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments	$ 2,000,000	$ 2,000,000