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Fair Value of Financial Instruments Fair Value Options Quantitative Disclosure (Details) - USD ($) | Sep. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Low [Member] |  |
Commodity Contracts |  |
Electricity physical forwards | $ 8.20 | $ 14.25
Natural gas financial swaps | 1.59 | 1.85
Financial futures - electricity | 8.20 | 8.57
High [Member] |  |
Commodity Contracts |  |
Electricity physical forwards | 37.15 | 54.73
Natural gas financial swaps | 3.22 | 4.92
Financial futures - electricity | 29.50 | 33.60
Weighted Average [Member] |  |
Commodity Contracts |  |
Electricity physical forwards | 28.36 | 38.18
Natural gas financial swaps | 2.07 | 2.64
Financial futures - electricity | 23.05 | 25.10
Assets [Member] |  |
Commodity Contracts |  |
Electricity physical forwards | 0 | 0
Natural gas financial swaps | 0 | 1,000,000
Financial futures - electricity | 0 | 1,000,000
Total commodity contracts | 0 | 2,000,000
Liabilities [Member] |  |
Commodity Contracts |  |
Electricity physical forwards | 140,000,000 | 112,000,000
Natural gas financial swaps | 13,000,000 | 9,000,000
Financial futures - electricity | 0 | 0
Total commodity contracts | $ 153,000,000 | $ 121,000,000

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