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Clearing Operations (Schedule of Derivative Contracts Outstanding Additional Information) (Details)
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9 Months Ended | |
|---|---|---|
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Sep. 30, 2013
TW
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Sep. 30, 2012
TW
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| Clearing Operations [Abstract] | ||
| Total volume in cleared power, in Terawatt hours (TWh) | 1,247 | 1,250 |