Fair Value Measurements - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash flow hedge gain (loss)	$ (4,000,000)	$ (2,000,000)
Derivative instruments	8,000,000	3,000,000
Designated as Hedging Instrument | Cash Flow Hedging | Interest Rate Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notional amount	92,000,000
Other Liabilities | Designated as Hedging Instrument | Cash Flow Hedging
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments	5,000,000	$ 2,000,000
EnerBank | Designated as Hedging Instrument | Fair Value Hedging | Interest Rate Swap, Notes Receivable
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notional amount	134,000,000
EnerBank | Other Liabilities | Designated as Hedging Instrument | Fair Value Hedging
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments	$ 1,000,000