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Subsequent Event (Details) - Deer Creek Ranch Water Co., LLC [Member] - SJWTX, Inc. [Member] - Subsequent Event [Member] $ in Thousands | Jul. 02, 2018 USD ($) a service_connection
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Subsequent Event [Line Items] |
Payment to acquire business | $ | $ 2,700
Number of service connections acquired | service_connection | 750
Area of land acquired (in acres) | a | 1,191

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- Definition Business Combination, Area Of Land Acquired + References No definition available. + Details
Name: | sjw_BusinessCombinationAreaOfLandAcquired
Namespace Prefix: | sjw_
Data Type: | num:areaItemType
Balance Type: | na
Period Type: | instant

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- Definition Business Combination, Number Of Service Connections Acquired + References No definition available. + Details
Name: | sjw_BusinessCombinationNumberOfServiceConnectionsAcquired
Namespace Prefix: | sjw_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Definition The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3213-108585 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event. + References No definition available. + Details
Name: | us-gaap_SubsequentEventLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_BusinessAcquisitionAxis=sjw_DeerCreekRanchWaterCo.LLCMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | dei_LegalEntityAxis=sjw_SJWTXInc.Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |