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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, 2012
Btu
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Dec. 31, 2011
Btu
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Energy [Member]
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| Derivative [Line Items] | ||||||
| Derivative, Nonmonetary Notional Amount | 465,396,010 | [1] | 240,363,364 | [1] | ||
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Temperature [Member]
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| Derivative [Line Items] | ||||||
| Derivative, Notional Amount per Unit | 21,596,501 | [1] | 14,917,438 | [1] | ||
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Agriculture [Member]
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| Derivative [Line Items] | ||||||
| Derivative, Nonmonetary Notional Amount | 13,073,000 | [1] | 6,098,000 | [1] | ||
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Precipitation [Member]
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| Derivative [Line Items] | ||||||
| Derivative, Notional Amount per Unit | 3,174,114 | [1] | 65,000 | [1] | ||
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Wind [Member]
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| Derivative [Line Items] | ||||||
| Derivative, Notional Amount per Unit | 513 | [1] | 712 | [1] | ||
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