Long-Term Debt Narrative (Details)	12 Months Ended
Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Unamortized Debt Issuance Expense	$ (3,908,000)	$ (4,257,000)
Long-term debt	435,000,000	437,717,000
Restricted Cash and Cash Equivalents	0	19,001,000	$ 0
Significant Observable Inputs (Level 2)
Fair value of long-term debt	537,840,000	502,446,000
Senior Notes [Member]
Long-term debt	315,000,000	315,000,000
San Jose Water Company [Member] \| Public Utility, Bonds [Member] \| California Pollution Control Financing Authority Revenue Bonds Two [Member]
Long-term debt	$ 70,000,000	70,000,000
Stated interest rates	4.75%
Restricted Cash and Cash Equivalents		19,001,000
San Jose Water Company [Member] \| Senior Notes [Member]
Ratio of indebtedness to total capital	0.6666
Ratio of net income available for interest charges for the trailing 12-month calendar period to interest charges	1.75
San Jose Water Company [Member] \| Senior Notes [Member] \| Series A [Member]
Long-term debt	$ 20,000,000	20,000,000
Stated interest rates	8.58%
San Jose Water Company [Member] \| Senior Notes [Member] \| Series L [Member]
Long-term debt	$ 50,000,000	50,000,000
Stated interest rates	5.14%
SJW Group [Member] \| Senior Notes [Member] \| Series A [Member]
Long-term debt	$ 50,000,000	50,000,000
Stated interest rates	4.35%
Ratio of indebtedness to total capital	0.6666
Minimum net worth required for compliance	$ 175,000,000
Cumulative net income, percentage included in net worth for compliance	30.00%
SJW Group [Member] \| Senior Notes [Member] \| Series A [Member] \| Financial Guarantee [Member]
Ratio of indebtedness to total capital	0.6666
Minimum net worth required for compliance	$ 125,000,000
SJW Group [Member] \| Senior Notes [Member] \| Series A [Member] \| Financial Guarantee [Member] \| Minimum [Member]
Cumulative net income, percentage included in net worth for compliance	30.00%
SJWTX,Inc. dba Canyon Lake Water Supply Corporation [Member] \| Senior Notes [Member] \| Series A [Member]
Long-term debt	$ 15,000,000	15,000,000
Stated interest rates	6.27%
Ratio of indebtedness to total capital	0.6666
Ratio of net income available for interest charges for the trailing 12-month calendar period to interest charges	1.75
Other Assets [Member] \| Accounting Standards Update 2015-03 [Member]
Unamortized Debt Issuance Expense	$ 3,561,000	$ 3,638,000

Cumulative Net Income, Percentage Included in Net Worth for Compliance

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Ratio of Income to Interest Charges

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Ratio of Indebtedness to Total Capital

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

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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.

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The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

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The lowest amount of net worth (total assets minus total liabilities) required by secondary market investors or state-imposed regulatory mandates.

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Amount of cash and cash equivalents restricted as to withdrawal or usage. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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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.

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