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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 |

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