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Summary of Significant Accounting Policies (Asset Retirement Obligation) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Retirement obligation	$ 5,231	$ 4,835
Discount rate	6.00%	6.00%
Present value, recorded as a liability	$ 1,184	$ 1,966
Deferred tax	0	1,352
Regulatory asset	$ 1,184	$ 3,318

X

- Definition

Asset Retirement Obligation, Deferred Tax

+ References

No definition available.

+ Details

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X

- Definition

Asset Retirement Obligation, Discount Rate

+ References

No definition available.

+ Details

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X

- Definition

Asset Retirement Obligation, Present Value

+ References

No definition available.

+ Details

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Namespace Prefix:	sjw_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- References

No definition available.

+ Details

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X

- Definition

The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

+ Details

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Period Type:	instant

X

- Definition

Carrying amount of an asset that is legally restricted for purposes of settling an asset retirement obligation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

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