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Benefit Plans Regulatory Asset (Details) - Postretirement pensions and other medical benefits [Member] - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
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Defined Benefit Plan Disclosure [Line Items] |  |
Funded status of obligation | $ 74,046 | $ 71,297
Accrued benefit cost | (5,490) | (6,239)
Amount to be recovered in future rates | 68,556 | 65,058
Tax gross-up | 0 | 44,737
Regulatory asset | $ 68,556 | $ 109,795

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- Definition Regulatory Assets, Amount to be Recovered in Future Rates + References No definition available. + Details
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Namespace Prefix: | sjw_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Regulatory Assets, Funded Status of Obligation + References No definition available. + Details
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Namespace Prefix: | sjw_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Regulatory Assets, Tax Gross Up + References No definition available. + Details
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Namespace Prefix: | sjw_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Regulatory Liabilities, Accrued Benefit Cost + References No definition available. + Details
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Namespace Prefix: | sjw_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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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. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
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- Definition The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |