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Fair Value of Financial Instruments Financial Assets and Liabilities Recognized at Fair Value (Details) - USD ($) $ in Millions | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Assets: |  |
Cash equivalents | $ 170 | $ 30
Debt securities: |  |
Domestic government | 24 | 11
Corporate credit | 12 | 6
Money market funds measured at NAV (2) | 6 | 25
Non-qualified benefit plan trust: (2) |  |
Money market funds | 3 | 1
Eqauity securities - domestic | 6 | 7
Debt securities—domestic government | 1 | 1
Assets from price risk management activities: (1) (3) |  |
Electricity | 5 | 3
Natural gas | 1 | 3
Total | 6 | 25
Assets, Fair Value Disclosure | 234 | 87
Liabilities from price risk management activities: (1) (3) |  |
Electricity | 116 | 135
Natural gas | 50 | 75
Total | 166 | 210
Fair Value, Inputs, Level 1 [Member] |  |
Assets: |  |
Cash equivalents | 170 | 30
Debt securities: |  |
Domestic government | 8 | 4
Corporate credit | 0 | 0
Non-qualified benefit plan trust: (2) |  |
Money market funds | 3 | 1
Eqauity securities - domestic | 6 | 7
Debt securities—domestic government | 1 | 1
Assets from price risk management activities: (1) (3) |  |
Electricity | 0 | 0
Natural gas | 0 | 0
Total | 188 | 43
Liabilities from price risk management activities: (1) (3) |  |
Electricity | 0 | 0
Natural gas | 0 | 0
Total | 0 | 0
Fair Value, Inputs, Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative Asset | 6 |
Assets: |  |
Cash equivalents | 0 | 0
Debt securities: |  |
Domestic government | 16 | 7
Corporate credit | 12 | 6
Non-qualified benefit plan trust: (2) |  |
Money market funds | 0 | 0
Eqauity securities - domestic | 0 | 0
Debt securities—domestic government | 0 | 0
Assets from price risk management activities: (1) (3) |  |
Electricity | 4 | 3
Natural gas | 1 | 3
Total | 39 | 19
Liabilities from price risk management activities: (1) (3) |  |
Electricity | 4 | 5
Natural gas | 36 | 66
Total | 40 | 71
Fair Value, Inputs, Level 3 [Member] |  |
Assets: |  |
Cash equivalents | 0 | 0
Debt securities: |  |
Domestic government | 0 | 0
Corporate credit | 0 | 0
Non-qualified benefit plan trust: (2) |  |
Money market funds | 0 | 0
Eqauity securities - domestic | 0 | 0
Debt securities—domestic government | 0 | 0
Assets from price risk management activities: (1) (3) |  |
Electricity | 1 | 0
Natural gas | 0 | 0
Total | 1 | 0
Liabilities from price risk management activities: (1) (3) |  |
Electricity | 112 | 130
Natural gas | 14 | 9
Total | $ 126 | $ 139

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