Long-Term Debt Long-term Debt Schedule (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Long-term debt	$ 435,000	$ 437,717
Unamortized Debt Issuance Expense	3,908	4,257
Current portion	0	125
Long-term debt, less current portion	431,092	433,335
Senior Notes [Member]
Debt Instrument [Line Items]
Long-term debt	315,000	315,000
Mortgages [Member] \| 444 West Santa Clara Street, L.P. [Member]
Debt Instrument [Line Items]
Long-term debt	$ 0	2,717
Stated interest rates	5.68%
San Jose Water Company [Member] \| Senior Notes [Member] \| Series A [Member]
Debt Instrument [Line Items]
Long-term debt	$ 20,000	20,000
Stated interest rates	8.58%
San Jose Water Company [Member] \| Senior Notes [Member] \| Series B [Member]
Debt Instrument [Line Items]
Long-term debt	$ 30,000	30,000
Stated interest rates	7.37%
San Jose Water Company [Member] \| Senior Notes [Member] \| Series C [Member]
Debt Instrument [Line Items]
Long-term debt	$ 10,000	10,000
Stated interest rates	9.45%
San Jose Water Company [Member] \| Senior Notes [Member] \| Series D [Member]
Debt Instrument [Line Items]
Long-term debt	$ 15,000	15,000
Stated interest rates	7.15%
San Jose Water Company [Member] \| Senior Notes [Member] \| Series E [Member]
Debt Instrument [Line Items]
Long-term debt	$ 15,000	15,000
Stated interest rates	6.81%
San Jose Water Company [Member] \| Senior Notes [Member] \| Series F [Member]
Debt Instrument [Line Items]
Long-term debt	$ 20,000	20,000
Stated interest rates	7.20%
San Jose Water Company [Member] \| Senior Notes [Member] \| Series G [Member]
Debt Instrument [Line Items]
Long-term debt	$ 20,000	20,000
Stated interest rates	5.93%
San Jose Water Company [Member] \| Senior Notes [Member] \| Series H [Member]
Debt Instrument [Line Items]
Long-term debt	$ 20,000	20,000
Stated interest rates	5.71%
San Jose Water Company [Member] \| Senior Notes [Member] \| Series I [Member]
Debt Instrument [Line Items]
Long-term debt	$ 20,000	20,000
Stated interest rates	5.93%
San Jose Water Company [Member] \| Senior Notes [Member] \| Series J [Member]
Debt Instrument [Line Items]
Long-term debt	$ 10,000	10,000
Stated interest rates	6.54%
San Jose Water Company [Member] \| Senior Notes [Member] \| Series K [Member]
Debt Instrument [Line Items]
Long-term debt	$ 20,000	20,000
Stated interest rates	6.75%
San Jose Water Company [Member] \| Senior Notes [Member] \| Series L [Member]
Debt Instrument [Line Items]
Long-term debt	$ 50,000	50,000
Stated interest rates	5.14%
San Jose Water Company [Member] \| Public Utility, Bonds [Member] \| California Pollution Control Financing Authority Revenue Bonds [Member]
Debt Instrument [Line Items]
Long-term debt	$ 50,000	50,000
Stated interest rates	5.10%
San Jose Water Company [Member] \| Public Utility, Bonds [Member] \| California Pollution Control Financing Authority Revenue Bonds Two [Member]
Debt Instrument [Line Items]
Long-term debt	$ 70,000	70,000
Stated interest rates	4.75%
SJW Group [Member] \| Senior Notes [Member] \| Series A [Member]
Debt Instrument [Line Items]
Long-term debt	$ 50,000	50,000
Stated interest rates	4.35%
SJWTX,Inc. dba Canyon Lake Water Supply Corporation [Member] \| Senior Notes [Member] \| Series A [Member]
Debt Instrument [Line Items]
Long-term debt	$ 15,000	$ 15,000
Stated interest rates	6.27%

Contractual interest rate for funds borrowed, under the debt agreement.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

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

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_LongTermDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19,20)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_LongTermDebtCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_LongTermDebtNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_UnamortizedDebtIssuanceExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant