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Risk Management Activities (Commodity Price Risk Program Derivative Contracted Gross Volumes) (Details)
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6 Months Ended | 12 Months Ended | ||||||
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Jun. 30, 2013
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Dec. 31, 2012
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Gas Price Volatility Program [Member]
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| Derivative [Line Items] | ||||||||
| Commodity price risk program derivative contracted gross volumes, MMDth | 23,900,000 | 26,300,000 | ||||||
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Price Protection Service Program [Member]
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| Derivative [Line Items] | ||||||||
| Commodity price risk program derivative contracted gross volumes, MMDth | 700,000 | 1,200,000 | ||||||
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DependaBill Program [Member]
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| Derivative [Line Items] | ||||||||
| Commodity price risk program derivative contracted gross volumes, MMDth | 200,000 | 300,000 | ||||||
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Gas Marketing Program [Member]
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| Derivative [Line Items] | ||||||||
| Commodity price risk program derivative contracted gross volumes, MMDth | 3,900,000 | [1] | 9,100,000 | [1] | ||||
| Basis contract volumes, MMDth | 3,500,000 | 8,200,000 | ||||||
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Gas Marketing Forward Physical Derivatives [Member]
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| Derivative [Line Items] | ||||||||
| Commodity price risk program derivative contracted gross volumes, MMDth | 3,800,000 | [2] | 8,400,000 | [2] | ||||
| Basis contract volumes, MMDth | 4,000,000 | 9,200,000 | ||||||
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Electric Energy Program [Member]
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| Derivative [Line Items] | ||||||||
| Commodity price risk program derivative contracted gross volumes, MMDth | 3,026,700,000 | 8,927,300,000 | ||||||
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