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Summary of Significant Accounting Policies - Narrative (Details) service_connection in Thousands, people in Thousands | 12 Months Ended
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Dec. 31, 2020 USD ($) mi² entity service_connection people subsidiary community | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($)
Public Utilities, General Disclosures [Line Items] |  |  |
Number of variable interest entities | entity | 2 |  |
Impairment of Long-Lived Assets Held-for-use | $ | $ 0 | $ 0 | $ 0
Goodwill, Impairment Loss | $ | $ 0 | $ 0 | $ 0
Minimum |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Service area in square miles | mi² | 247 |  |
Minimum | Finite-Lived Intangible Assets |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Estimated useful life | 7 years |  |
Maximum | Finite-Lived Intangible Assets |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Estimated useful life | 12 years |  |
Revenues | Product Concentration Risk |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Percentage of revenue derived from sales of water to business and residential customers | 93.00% |  |
San Jose Water Company |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Public Utilities, Service Connections | service_connection | 231 |  |
Number of people served | people | 1,000,000,000 |  |
Connecticut Water Service, Inc. |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Number of people served | people | 485 |  |
Public Utilities, Number of Communities Served | community | 81 |  |
Acequia Water Supply Corporation |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Ownership percentage in Acequia Water Supply Corporation | 25.00% |  |
444 West Santa Clara Street, L.P. | 444 West Santa Clara Street, L.P. |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Limited partnership interest percentage | 70.00% |  |
San Jose Water Company |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Number of subsidiaries that are primary beneficiaries | subsidiary | 2 |  |
SJWTX, Inc. dba Canyon Lake Water Supply Corporation |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Number of people served | people | 59 |  |
Connecticut and Maine | Connecticut Water Service, Inc. |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Public Utilities, Service Connections | service_connection | 139 |  |
Connecticut | Connecticut Water Service, Inc. |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Public Utilities, Service Connections | service_connection | 3 |  |

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