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Assets and Liabilities with Recurring Fair Value Measurements Quantitative Info on Level 3 (Details) (USD $)
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Jun. 30, 2013
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Dec. 31, 2012
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Minimum [Member]
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| Fair Value Inputs, Assets, Quantitative Information [Line Items] | ||
| Fair Value Inputs Quantitative Information Power | $ 31.00 | $ 23.75 |
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Maximum [Member]
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| Fair Value Inputs, Assets, Quantitative Information [Line Items] | ||
| Fair Value Inputs Quantitative Information Power | 50.00 | 53.82 |
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Physical Power [Member]
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| Fair Value Inputs, Assets, Quantitative Information [Line Items] | ||
| Fair Value Inputs Quantitative Information Power | $ 6,000,000 | $ 11,000,000 |