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Fair Value of Financial Instruments Fair Value Options Quantitative Disclosure (Details) - USD ($) | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Low [Member] |  |
Commodity Contracts |  |
Electricity physical forwards | $ 12.34 | $ 7.79
Natural gas financial swaps | 1.02 | 1.26
Financial futures - electricity | 14.90 | 7.79
High [Member] |  |
Commodity Contracts |  |
Electricity physical forwards | 49.11 | 41.23
Natural gas financial swaps | 3.21 | 2.92
Financial futures - electricity | 30.75 | 29.74
Weighted Average [Member] |  |
Commodity Contracts |  |
Electricity physical forwards | 33.83 | 30.95
Natural gas financial swaps | 1.68 | 1.90
Financial futures - electricity | 25.15 | 21.74
Assets [Member] |  |
Commodity Contracts |  |
Electricity physical forwards | 0 | 0
Natural gas financial swaps | 0 | 0
Financial futures - electricity | 1,000,000 | 0
Total commodity contracts | 1,000,000 | 0
Liabilities [Member] |  |
Commodity Contracts |  |
Electricity physical forwards | 112,000,000 | 130,000,000
Natural gas financial swaps | 14,000,000 | 9,000,000
Financial futures - electricity | 0 | 0
Total commodity contracts | $ 126,000,000 | $ 139,000,000

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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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