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Financial Statement Schedule - Additional Information (Details) $ in Thousands	Dec. 31, 2020 USD ($)
Condensed Financial Information Disclosure [Abstract]
Restricted net assets, subsidiaries	$ 283,807
Restricted net assets, subsidiaries, percent of consolidated assets	31.00%

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- Definition

Restricted Net Assets, Subsidiaries

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No definition available.

+ Details

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Balance Type:	debit
Period Type:	instant

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- Definition

Restricted Net Assets, Subsidiaries, Percent Of Consolidated Assets

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No definition available.

+ Details

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Namespace Prefix:	sjw_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

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- References

No definition available.

+ Details

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Namespace Prefix:	srt_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration