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Derivative and Financial Instruments - Commodity Notional (Details 2)
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Mar. 31, 2013
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Dec. 31, 2012
ft3
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Natural gas and power, Fixed price [Member]
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| Trading Activity, Gains and Losses, Net [Line Items] | ||
| Commodity derivatives - volumetric material net exposures | (22,000,000,000) | (48,000,000,000) |
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Natural gas and power, Basis [Member]
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| Trading Activity, Gains and Losses, Net [Line Items] | ||
| Commodity derivatives - volumetric material net exposures | (1,000,000,000) | 125,000,000,000 |