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Fair Value of FInancial Instruments Fair Value Options Quantitative Disclosure (Details) - USD ($) | Dec. 31, 2017 | Dec. 31, 2016
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Minimum [Member] |  |
Fair Value, Option, Quantitative Disclosures [Line Items] |  |
Electricity physical forward purchase | $ 7.79 | $ 14.25
Natural gas financial swaps | 1.26 | 1.85
Fnancial swaps - electricity | 7.79 | 8.57
Maximum [Member] |  |
Fair Value, Option, Quantitative Disclosures [Line Items] |  |
Electricity physical forward purchase | 41.23 | 54.73
Natural gas financial swaps | 2.92 | 4.92
Fnancial swaps - electricity | 29.74 | 33.60
Weighted Average [Member] |  |
Fair Value, Option, Quantitative Disclosures [Line Items] |  |
Electricity physical forward purchase | 30.95 | 38.18
Natural gas financial swaps | 1.90 | 2.64
Fnancial swaps - electricity | 21.74 | 25.10
Assets [Member] |  |
Fair Value, Option, Quantitative Disclosures [Line Items] |  |
Electricity physical forward purchase | 0 | 0
Natural gas financial swaps | 0 | 1,000,000
Fnancial swaps - electricity | 0 | 1,000,000
Total commodity contracts | 0 | 2,000,000
Liabilities [Member] |  |
Fair Value, Option, Quantitative Disclosures [Line Items] |  |
Electricity physical forward purchase | 130,000,000 | 112,000,000
Natural gas financial swaps | 9,000,000 | 9,000,000
Fnancial swaps - electricity | 0 | 0
Total commodity contracts | $ 139,000,000 | $ 121,000,000

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- Definition Electricity physical forward purchase + References No definition available. + Details
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- Definition Fnancial swaps - electricity + References No definition available. + Details
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- Definition Natural gas financial swaps + References No definition available. + Details
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