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Derivative Instruments (Details) $ in Thousands | 12 Months Ended
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Dec. 31, 2016 USD ($) MWh quote | Dec. 31, 2015 USD ($)
Purchase power contract |  |
Derivative instruments |  |
Number of broker quotes received to determine fair value of derivative instrument | quote | 1 |
GSWC |  |
Derivative instruments |  |
Derivative Activity Volume | MWh | 352,000 |
GSWC | Purchase power contract |  |
Changes in the fair value of the derivative |  |
Balance, at beginning of the period | $ (7,053) | $ (3,339)
Unrealized gain (loss) on purchased power contracts | 2,152 | (3,714)
Balance, at end of the period | $ (4,901) | $ (7,053)
Minimum | GSWC | Purchase power contract |  |
Derivative instruments |  |
Derivative, Term of Contract | 3 years |
Maximum [Member] | GSWC | Purchase power contract |  |
Derivative instruments |  |
Derivative, Term of Contract | 5 years |

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- Definition Period the derivative contract is outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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