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Derivative Instruments (Details) MWh in Thousands, $ in Thousands	12 Months Ended
	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

X

- Definition

Derivative Activity Volume

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- Definition

Number of Broker Quotes Received to Determine Fair Value of Derivative Instrument

+ References

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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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

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

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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

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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

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