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Fair Value of FInancial Instruments Schedule of Fair Value (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Domestic government, Debt securities | $ 11 | $ 12
Corporate debt securities held in decommissioning trust assets | 6 | 8
Money market funds | 25 | 21
Fair Value - Money market funds | 1 | 1
Domestic Equity Securities | 7 | 4
Debt securities - domestic government | 1 | 1
Investments, Fair Value Disclosure | 0 | 2
Electricity | 3 | 7
Natural gas | 3 | 16
Financial Instruments, Owned, at Fair Value | 25 | 23
Assets, Fair Value Disclosure | 57 | 72
Liabilities from price risk management activities: [Abstract] |  |
Electricity | 135 | 118
Natural gas | 75 | 51
Liabilities, Fair Value Disclosure | 210 | 169
Fair Value, Inputs, Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Domestic government, Debt securities | 4 | 2
Corporate debt securities held in decommissioning trust assets | 0 | 0
Fair Value - Money market funds | 1 | 1
Domestic Equity Securities | 7 | 4
Debt securities - domestic government | 1 | 1
Electricity | 0 | 0
Natural gas | 0 | 0
Financial Instruments, Owned, at Fair Value | 13 | 8
Liabilities from price risk management activities: [Abstract] |  |
Electricity | 0 | 0
Natural gas | 0 | 0
Liabilities, Fair Value Disclosure | 0 | 0
Fair Value, Inputs, Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Domestic government, Debt securities | 7 | 10
Corporate debt securities held in decommissioning trust assets | 6 | 8
Fair Value - Money market funds | 0 | 0
Domestic Equity Securities | 0 | 0
Debt securities - domestic government | 0 | 0
Electricity | 3 | 6
Natural gas | 3 | 15
Financial Instruments, Owned, at Fair Value | 19 | 39
Liabilities from price risk management activities: [Abstract] |  |
Electricity | 5 | 6
Natural gas | 66 | 42
Liabilities, Fair Value Disclosure | 71 | 48
Fair Value, Inputs, Level 3 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Domestic government, Debt securities | 0 | 0
Corporate debt securities held in decommissioning trust assets | 0 | 0
Money market funds | 0 |
Fair Value - Money market funds |  | 0
Domestic Equity Securities | 0 | 0
Debt securities - domestic government | 0 | 0
Electricity | 0 | 1
Natural gas | 0 | 1
Financial Instruments, Owned, at Fair Value | 0 | 2
Liabilities from price risk management activities: [Abstract] |  |
Electricity | 130 | 112
Natural gas | 9 | 9
Liabilities, Fair Value Disclosure | 139 | 121
Non Qualified Benefit Plans [Member] |  |
Liabilities from price risk management activities: [Abstract] |  |
Insurance contracts, at cash surrender value | $ 28 | $ 26

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