Authorized Regulatory Base Charge, Per Residential Account

No definition available.

Name:	sjw_AuthorizedRegulatoryBaseChargePerResidentialAccount
Namespace Prefix:	sjw_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Authorized Regulatory Surcharge, Per Thousand Gallons

+ References

No definition available.

+ Details

Name:	sjw_AuthorizedRegulatorySurchargePerThousandGallons
Namespace Prefix:	sjw_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Authorized WICA Surcharge, Percentage

+ References

No definition available.

+ Details

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

- Definition

Balancing and Memorandum Revenue Account, Increase (Decrease)

+ References

No definition available.

+ Details

Name:	sjw_BalancingandMemorandumRevenueAccountIncreaseDecrease
Namespace Prefix:	sjw_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

CPED Calculated Double-Billing Refund

+ References

No definition available.

+ Details

Name:	sjw_CPEDCalculatedDoubleBillingRefund
Namespace Prefix:	sjw_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

CPED Proposed Service Charge Refund

+ References

No definition available.

+ Details

Name:	sjw_CPEDCalculatedServiceChargeRefund
Namespace Prefix:	sjw_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

CPED Proposed Penalty, Per Offense, Per Day

+ References

No definition available.

+ Details

Name:	sjw_CPEDProposedPenaltyPerOffensePerDay
Namespace Prefix:	sjw_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Customer Credit Refund

+ References

No definition available.

+ Details

Name:	sjw_CustomerCreditRefund
Namespace Prefix:	sjw_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Memorandum Account, Requested Recovery (Refund)

+ References

No definition available.

+ Details

Name:	sjw_MemorandumAccountRequestedRecoveryRefund
Namespace Prefix:	sjw_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Memorandum Account, Requested Recovery (Refund), Monthly Amount

+ References

No definition available.

+ Details

Name:	sjw_MemorandumAccountRequestedRecoveryRefundMonthlyAmount
Namespace Prefix:	sjw_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Public Utilities, Approved Rate Increase (Decrease), Percentage, Step Increase 1

+ References

No definition available.

+ Details

Name:	sjw_PublicUtilitiesApprovedRateIncreaseDecreasePercentageStepIncrease1
Namespace Prefix:	sjw_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Public Utilities, Approved Rate Increase (Decrease), Percentage, Step Increase 2

+ References

No definition available.

+ Details

Name:	sjw_PublicUtilitiesApprovedRateIncreaseDecreasePercentageStepIncrease2
Namespace Prefix:	sjw_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Regulatory Rate Filings, Proposed Rate Increase, Percent of Authorized Revenue, Year Two

+ References

No definition available.

+ Details

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

- Definition

Regulatory Rate Filings, Requested Deferment Period

+ References

No definition available.

+ Details

Name:	sjw_RegulatoryRateFilingsRequestedDefermentPeriod
Namespace Prefix:	sjw_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Regulatory Rate Filings, Requested Rate Increase as Percentage of Total Revenue at Time of Request, Year Three

+ References

No definition available.

+ Details

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

- Definition

Regulatory Rate Filings, Requested Rate Increase, Percent of Authorized Revenue, Year Four

+ References

No definition available.

+ Details

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

- Definition

Regulatory Rate Filings, Requested Rate Increase, Year Four

+ References

No definition available.

+ Details

Name:	sjw_RegulatoryRateFilingsRequestedRateIncreaseYearFour
Namespace Prefix:	sjw_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Regulatory Rate Filings, Requested Rate Increase, Year One

+ References

No definition available.

+ Details

Name:	sjw_RegulatoryRateFilingsRequestedRateIncreaseYearOne
Namespace Prefix:	sjw_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Regulatory Rate Filings, Requested Rate Increase, Year Three

+ References

No definition available.

+ Details

Name:	sjw_RegulatoryRateFilingsRequestedRateIncreaseYearThree
Namespace Prefix:	sjw_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Regulatory Rate Filings, Requested Rate Increase, Year Two

+ References

No definition available.

+ Details

Name:	sjw_RegulatoryRateFilingsRequestedRateIncreaseYearTwo
Namespace Prefix:	sjw_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Regulatory Rate Filings, Requested Rate Increase as Percentage of Total Revenue at Time of Request, Year One

+ References

No definition available.

+ Details

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

- Definition

Requested WICA Surcharge Increase (Decrease), Percentage

+ References

No definition available.

+ Details

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

- Definition

Requested WICA Surcharge, Percentage

+ References

No definition available.

+ Details

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

- Definition

Requested WISC Surcharge Increase (Decrease), Percentage

+ References

No definition available.

+ Details

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

- Definition

Utility Plant Investment Commitment

+ References

No definition available.

+ Details

Name:	sjw_UtilityPlantInvestmentCommitment
Namespace Prefix:	sjw_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of public utility's approved rate increase (decrease) by regulatory agency.

+ References

No definition available.

+ Details

Name:	us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Percentage of public utility's approved rate increase (decrease) by regulatory agency.

+ References

No definition available.

+ Details

Name:	us-gaap_PublicUtilitiesApprovedRateIncreaseDecreasePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- 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_PublicUtilitiesGeneralDisclosuresLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of public utility's requested rate increase (decrease) with regulatory agency.

+ References

No definition available.

+ Details

Name:	us-gaap_PublicUtilitiesRequestedRateIncreaseDecreaseAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Percentage of public utility's requested rate increase (decrease) with regulatory agency.

+ References

No definition available.

+ Details

Name:	us-gaap_PublicUtilitiesRequestedRateIncreaseDecreasePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_PublicUtilitiesRegulatoryProceedingAxis=sjw_OIINo.1809003SettlementAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=sjw_AvonWaterCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=sjw_SanJoseWaterCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PublicUtilitiesRegulatoryProceedingAxis=sjw_CPEDCalculatedRefund20142016Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PublicUtilitiesRegulatoryProceedingAxis=sjw_CPEDCalculatedRefund19872013Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PublicUtilitiesRegulatoryProceedingAxis=sjw_AdviceLetterNo.545Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PublicUtilitiesRegulatoryProceedingAxis=sjw_AdviceLetterNo.534Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PublicUtilitiesRegulatoryProceedingAxis=sjw_AdviceLetterNo.548Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PublicUtilitiesRegulatoryProceedingAxis=sjw_AdviceLetterNo.537Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PublicUtilitiesRegulatoryProceedingAxis=sjw_AdviceLetterNo.541Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PublicUtilitiesRegulatoryProceedingAxis=sjw_ApplicationNo.1912002Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PublicUtilitiesRegulatoryProceedingAxis=sjw_CostOfCapitalFilingsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=sjw_ConnecticutWaterCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	dei_LegalEntityAxis=sjw_MaineWaterCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RegulatoryAssetAxis=sjw_CostofcapitalMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_PublicUtilitiesRegulatoryProceedingAxis=sjw_WICAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Regulatory Rate Filings (Details) - USD ($)													3 Months Ended		9 Months Ended
Regulatory Rate Filings (Details) - USD ($)	Oct. 28, 2020	Aug. 27, 2020	Jun. 17, 2020	Jan. 22, 2020	Jan. 20, 2020	Dec. 20, 2019	Nov. 20, 2019	Oct. 01, 2019	Sep. 30, 2019	Feb. 01, 2019	Jan. 28, 2019	Sep. 14, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Apr. 24, 2020	Feb. 07, 2020	Jan. 28, 2020	Dec. 06, 2019	Oct. 18, 2019	Aug. 01, 2019	Jul. 26, 2019
Public Utilities, General Disclosures [Line Items]
Regulatory Asset Increase (Decrease)													$ (1,087,000)	$ (9,275,000)	$ 2,299,000	$ (5,410,000)
OII No. 18-09-003 Settlement Agreement
Public Utilities, General Disclosures [Line Items]
Customer Credit Refund																							$ 350,000
OII No. 18-09-003 Settlement Agreement \| Minimum
Public Utilities, General Disclosures [Line Items]
Customer Credit Refund																							1,757,000
Avon Water Company [Member]
Public Utilities, General Disclosures [Line Items]
Authorized WICA Surcharge, Percentage													9.31%		9.31%
San Jose Water Company [Member]
Public Utilities, General Disclosures [Line Items]
CPED Calculated Double-Billing Refund												$ 4,935,000
San Jose Water Company [Member] \| Minimum
Public Utilities, General Disclosures [Line Items]
CPED Proposed Penalty, Per Offense, Per Day												500
San Jose Water Company [Member] \| Maximum
Public Utilities, General Disclosures [Line Items]
CPED Proposed Penalty, Per Offense, Per Day												50,000
San Jose Water Company [Member] \| CPED Calculated Refund 2014-2016
Public Utilities, General Disclosures [Line Items]
CPED Calculated Service Charge Refund												2,061,000
San Jose Water Company [Member] \| CPED Calculated Refund 1987-2013
Public Utilities, General Disclosures [Line Items]
CPED Calculated Service Charge Refund												$ 1,990,000
San Jose Water Company [Member] \| OII No. 18-09-003 Settlement Agreement
Public Utilities, General Disclosures [Line Items]
Utility Plant Investment Commitment																							5,000,000
San Jose Water Company [Member] \| OII No. 18-09-003 Settlement Agreement \| Maximum
Public Utilities, General Disclosures [Line Items]
Customer Credit Refund																							2,100,000
San Jose Water Company [Member] \| Advice Letter No. 545 [Member]
Public Utilities, General Disclosures [Line Items]
Customer Credit Refund																							$ (5.52)
San Jose Water Company [Member] \| Advice Letter No. 534 [Member]
Public Utilities, General Disclosures [Line Items]
Memorandum Account, Requested Recovery																						$ 1,243,000
San Jose Water Company [Member] \| Advice Letter No. 548 [Member]
Public Utilities, General Disclosures [Line Items]
Memorandum Account, Requested Recovery																	$ 1,224,000
Public Utilities, Approved Rate Increase (Decrease), Amount		$ 1,219,000
San Jose Water Company [Member] \| Advice Letter No. 537
Public Utilities, General Disclosures [Line Items]
Memorandum Account, Requested Refund, Monthly Amount																					$ 20.84
San Jose Water Company [Member] \| Advice Letter No. 541 [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount							$ 8,600,000
Public Utilities, Requested Rate Increase (Decrease), Percentage							2.28%
San Jose Water Company [Member] \| Application No. 19-12-002 [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory Rate Filings, Requested Rate Increase, Year One																				$ 2,315,000
Regulatory Rate Filings, Requested Rate Increase as Percentage of Total Revenue at Time of Request, Year One																				0.61%
Regulatory Rate Filings, Requested Rate Increase, Year Two																				$ 3,960,000
Regulatory Rate Filings, Proposed Rate Increase, Percent of Authorized Revenue, Year Two																				1.04%
Regulatory Rate Filings, Requested Rate Increase, Year Three																				$ 2,510,000
Regulatory Rate Filings, Requested Rate Increase, Percent of Authorized Revenue, Year Three																				0.65%
Regulatory Rate Filings, Requested Rate Increase, Year Four																				$ 341,000
Regulatory Rate Filings, Requested Rate Increase, Percent of Authorized Revenue, Year Four																				0.09%
San Jose Water Company [Member] \| Cost Of Capital Filings [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory Rate Filings, Requested Deferment Period				1 year
Connecticut Water Company [Member]
Public Utilities, General Disclosures [Line Items]
Authorized WICA Surcharge, Percentage													5.75%		5.75%
Requested WICA Surcharge Increase (Decrease), Percentage																		(0.09%)	2.60%
Requested WICA Surcharge, Percentage																		5.75%	5.84%
SJWTX, Inc. [Member]
Public Utilities, General Disclosures [Line Items]
Authorized Regulatory Surcharge, Per Thousand Gallons								$ 2.02	$ 2.19	$ 1.05	$ 0.95
Authorized Regulatory Base Charge, Per Residential Account								$ 3.04
Maine Water Company [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount					$ 371,000	$ 221,000
Public Utilities, Requested Rate Increase (Decrease), Percentage						14.70%
Public Utilities, Approved Rate Increase (Decrease), Amount			$ 198,000
Public Utilities, Approved Rate Increase (Decrease), Percentage, Step Increase 1			9.80%
Public Utilities, Approved Rate Increase (Decrease), Percentage, Step Increase 2			3.51%
Public Utilities, Approved Rate Increase (Decrease), Percentage			13.31%
Maine Water Company [Member] \| Minimum
Public Utilities, General Disclosures [Line Items]
Requested WISC Surcharge Increase (Decrease), Percentage					1.76%
Maine Water Company [Member] \| Maximum
Public Utilities, General Disclosures [Line Items]
Requested WISC Surcharge Increase (Decrease), Percentage					3.00%
Cost of capital memorandum account
Public Utilities, General Disclosures [Line Items]
Regulatory Asset Increase (Decrease)													$ (1,000)	$ (7,000)	$ (8,000)	$ (24,000)
Subsequent Event [Member] \| Connecticut Water Company [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 956,000
Requested WICA Surcharge, Percentage	6.85%
Subsequent Event [Member] \| Connecticut Water Company [Member] \| WICA [Member]
Public Utilities, General Disclosures [Line Items]
Requested WICA Surcharge Increase (Decrease), Percentage	1.11%