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Fair Value Measurements - Narrative (Details) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2021 | Mar. 31, 2020 | Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Cash flow hedge gain (loss) | $ 1 | $ (5) |
Derivative instruments | 14 |  | $ 17
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 | 83 |  | 85
Other Liabilities | Designated as Hedging Instrument | Cash Flow Hedging |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Derivative instruments | 7 |  | 9
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 |  | 134
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 | $ 5 |  | $ 6

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