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Regulatory Matters - Narrative (Details) - USD ($) $ in Thousands |  | 3 Months Ended
--- | --- | ---
Dec. 19, 2024 | Mar. 31, 2025
Regulated Operations [Abstract] |  |
Reduction in authorized return of equity |  | 0.20%
Authorized revenue, threshold percentage |  | 2.00%
Balancing and memorandum revenue account, refunds (collections) | $ 15,792 |

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- Definition Balancing Accounts, Increase (Decrease) In Authorized Return On Equity, Percentage + References No definition available. + Details
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Namespace Prefix: | sjw_
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- Definition Balancing And Memorandum Revenue Account, Refunds (Collections) + References No definition available. + Details
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Namespace Prefix: | sjw_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Threshold balance of regulatory balancing or memorandum accounts as a percentage of authorized revenue to trigger an account review. + References No definition available. + Details
Name: | sjw_RegulatoryBalancingAndMemorandumAccountsReviewAuthorizedRevenueThresholdPercentage
Namespace Prefix: | sjw_
Data Type: | dtr-types1:percentItemType
Balance Type: | na
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_RegulatedOperationsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration