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Derivative Instruments (Details) - Golden State Water Company MWh in Thousands, $ in Thousands | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2018 USD ($) quote | Sep. 30, 2017 USD ($) | Sep. 30, 2018 USD ($) MWh quote | Sep. 30, 2017 USD ($) | Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |  |
Description of derivative activity volume | MWh |  |  | 123 |  |
Commodity Contract |  |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |  |
Number of broker quotes received to determine fair value of derivative instrument | quote | 1 |  | 1 |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |
Fair value at beginning of the period | $ (1,710) | $ (4,493) | $ (2,941) | $ (4,901) |
Unrealized gain on purchased power contracts | 567 | 656 | 1,798 | 1,064 |
Fair value at end of the period | $ (1,143) | $ (3,837) | $ (1,143) | $ (3,837) |
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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