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Summary of Significant Accounting Policies (Narrative) (Details) $ in Thousands |  | 12 Months Ended
--- | --- | ---
Feb. 22, 2016 USD ($) | Dec. 31, 2017 mi² people_served
Acequia Water Supply Corporation [Member] |  |
Public Utilities General Disclosures [Line Items] |  |
Ownership percentage in Acequia Water Supply Corporation |  | 25.00%
Minimum [Member] |  |
Public Utilities General Disclosures [Line Items] |  |
Service area in square miles | mi² |  | 244
Sales [Member] | Product Concentration Risk [Member] |  |
Public Utilities General Disclosures [Line Items] |  |
Percentage of revenue derived from sales of water to business and residential customers |  | 92.00%
San Jose Water Company [Member] |  |
Public Utilities General Disclosures [Line Items] |  |
Number of people served |  | 1,000,000
444 West Santa Clara Street, L.P. [Member] |  |
Public Utilities General Disclosures [Line Items] |  |
Limited partership interest percentage |  | 70.00%
SJWTX,Inc. dba Canyon Lake Water Supply Corporation [Member] |  |
Public Utilities General Disclosures [Line Items] |  |
Number of people served |  | 42,000
Texas Water Alliance Alliance [Member] |  |
Public Utilities General Disclosures [Line Items] |  |
Proceeds from Sales of Business, Affiliate and Productive Assets | $ | $ 31,000 |

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