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Derivative Instruments (Details) - GOLDEN STATE WATER COMPANY MWh in Thousands, $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 USD ($) MWh quote | Mar. 31, 2017 USD ($) | Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Description of derivative activity volume | MWh | 175 |  |
Commodity Contract |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Number of broker quotes received to determine fair value of derivative instrument | quote | 1 |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |
Fair value at beginning of the period | $ (2,941) | $ (4,901) |
Unrealized gain (loss) on purchased power contracts | 316 | (559) |
Fair value at end of the period | $ (2,625) | $ (5,460) |
Minimum | Commodity Contract |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Term of derivative contract |  |  | 3 years
Maximum | Commodity Contract |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Term of derivative 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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- Definition Amount of unrealized gain (loss) recognized in the income statement for a financial instrument classified as derivative asset (liability) after deduction of derivative liability (asset), 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 (d) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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