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Derivative Instruments (Gross Derivative Contract Positions Outstanding Relating To Energy And Weather Derivatives Trading Activities) (Details)
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Mar. 31, 2013
MMBTU
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Dec. 31, 2012
MMBTU
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Energy [Member]
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| Derivative [Line Items] | ||||||
| Derivative, Nonmonetary Notional Amount | 106,602,407 | [1] | 107,521,592 | [1] | ||
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Temperature [Member]
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| Derivative [Line Items] | ||||||
| Derivative, Notional Amount Per Unit | 4,246,626 | [1] | 8,168,052 | [1] | ||
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Precipitation [Member]
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| Derivative [Line Items] | ||||||
| Derivative, Notional Amount Per Unit | 4,808,787 | [1] | 4,453,934 | [1] | ||
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