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Benefit Plans - Regulatory Asset (Details) - Postretirement pensions and other medical benefits - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
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Defined Benefit Plan Disclosure [Line Items] |  |
Funded status of obligation | $ 124,282 | $ 110,039
Accrued benefit cost | (28,723) | (36,514)
Regulatory asset, amount to be recovered in future rates | $ 95,559 | $ 73,525

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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 Liabilities, Accrued Benefit Cost + References No definition available. + Details
Name: | sjw_RegulatoryLiabilitiesAccruedBenefitCost
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
Name: | us-gaap_DefinedBenefitPlanDisclosureLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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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://fasb.org/us-gaap/role/ref/legacyRef -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
Name: | us-gaap_RegulatoryAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |