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Fair Value of Financial Instruments Financial Assets and Liabilities Recognized at Fair Value (Details) - USD ($) $ in Millions | Sep. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Debt securities: |  |
Domestic government | $ 11 | $ 12
Corporate credit | 7 | 8
Money market funds measured at NAV (2) | 23 | 21
Non-qualified benefit plan trust: (2) |  |
Money market funds | 2 | 1
Eqauity securities - domestic | 6 | 4
Debt securities—domestic government | 1 | 1
Collective trust-domestic equity measured at NAV (2) | 0 | 2
Assets from price risk management activities: (1) (3) |  |
Electricity | 3 | 7
Natural gas | 1 | 16
Total | 23 | 23
Assets, Fair Value Disclosure | 54 | 72
Liabilities from price risk management activities: (1) (3) |  |
Electricity | 143 | 118
Natural gas | 50 | 51
Total | 193 | 169
Fair Value, Inputs, Level 1 [Member] |  |
Debt securities: |  |
Domestic government | 3 | 2
Corporate credit | 0 | 0
Non-qualified benefit plan trust: (2) |  |
Money market funds | 2 | 1
Eqauity securities - domestic | 6 | 4
Debt securities—domestic government | 1 | 1
Assets from price risk management activities: (1) (3) |  |
Electricity | 0 | 0
Natural gas | 0 | 0
Total | 12 | 8
Liabilities from price risk management activities: (1) (3) |  |
Electricity | 0 | 0
Natural gas | 0 | 0
Total | 0 | 0
Fair Value, Inputs, Level 2 [Member] |  |
Debt securities: |  |
Domestic government | 8 | 10
Corporate credit | 7 | 8
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 | 3 | 6
Natural gas | 1 | 15
Total | 19 | 39
Liabilities from price risk management activities: (1) (3) |  |
Electricity | 3 | 6
Natural gas | 37 | 42
Total | 40 | 48
Fair Value, Inputs, Level 3 [Member] |  |
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 | 0 | 1
Natural gas | 0 | 1
Total | 0 | 2
Liabilities from price risk management activities: (1) (3) |  |
Electricity | 140 | 112
Natural gas | 13 | 9
Total | $ 153 | $ 121

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