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Price Risk Management Future Year Net Unrealized Gain/Loss Recorded at Balance Sheet Date Expected to Become Realized (Details) (Gain (Loss) on Derivatives [Member], USD $) In Millions | 6 Months Ended | 12 Months Ended | 42 Months Ended
--- | --- | --- | ---
Dec. 31, 2011 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2014
Gain (Loss) on Derivatives [Member] |  |  |  |  |  |
Commodity contracts: |  |  |  |  |  |
Electricity | $ 22 | $ 0 | $ 7 | $ 18 | $ 39 | $ 86
Natural gas | 56 | 1 | 10 | 46 | 97 | 210
Net unrealized loss | $ 78 | $ 1 | $ 17 | $ 64 | $ 136 | $ 296

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- Definition Commodity contracts: [Abstract] + Details
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- Definition Net unrealized gain or loss related to commodity contracts recorded as of the balance sheet date, expected to be realized in the statement of income in a future period as a result of the settlement of the underlying derivative instrument. + Details
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- Definition Net unrealized gain or loss recorded as of the balance sheet date related to commodity contracts for electricity, expected to be realized in the statement of income in a future period as a result of the settlement of the underlying derivative instrument. + Details
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- Definition Net unrealized gain or loss recorded as of the balance sheet date related to commodity contracts for natural gas, expected to be realized in the statement of income in a future period as a result of the settlement of the underlying derivative instrument. + Details
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