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Fair Value of Financial Instruments Fair Value Options Quantitative Disclosure (Details) - USD ($) | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Low [Member] |  |
Commodity Contracts |  |
Electricity physical forwards | $ 6.56 | $ 7.79
Natural gas financial swaps | 1.12 | 1.26
Financial futures - electricity | 6.56 | 7.79
High [Member] |  |
Commodity Contracts |  |
Electricity physical forwards | 44.31 | 41.23
Natural gas financial swaps | 2.58 | 2.92
Financial futures - electricity | 27.74 | 29.74
Weighted Average [Member] |  |
Commodity Contracts |  |
Electricity physical forwards | 31.44 | 30.95
Natural gas financial swaps | 1.59 | 1.90
Financial futures - electricity | 18.46 | 21.74
Assets [Member] |  |
Commodity Contracts |  |
Electricity physical forwards | 0 | 0
Natural gas financial swaps | 0 | 0
Financial futures - electricity | 0 | 0
Total commodity contracts | 0 | 0
Liabilities [Member] |  |
Commodity Contracts |  |
Electricity physical forwards | 122,000,000 | 130,000,000
Natural gas financial swaps | 11,000,000 | 9,000,000
Financial futures - electricity | 1,000,000 | 0
Total commodity contracts | $ 134,000,000 | $ 139,000,000

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