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Derivative Instruments (Details) MWh in Thousands, $ in Thousands | 12 Months Ended
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Dec. 31, 2017 USD ($) MWh quote | Dec. 31, 2016 USD ($)
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 | 201 |
GSWC | Purchase power contract |  |
Changes in the fair value of the derivative |  |
Balance, at beginning of the period | $ (4,901) | $ (7,053)
Unrealized gain on purchased power contracts | 1,960 | 2,152
Balance, at end of the period | $ (2,941) | $ (4,901)
Minimum [Member] | GSWC | Purchase power contract |  |
Derivative instruments |  |
Derivative, Term of Contract | 3 years |
Maximum | GSWC | Purchase power contract |  |
Derivative instruments |  |
Derivative, Term of Contract | 5 years |

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- Definition Number of Broker Quotes Received to Determine Fair Value of Derivative Instrument + References No definition available. + Details
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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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- Definition Amount of gain (loss) recognized on the income statement for financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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- Definition Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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