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Regulatory Rate Filings (Details) - USD ($) |  |  |  |  |  |  |  |  |  |  |  |  | 3 Months Ended | 9 Months Ended |  |  |  |  |  |  |
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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% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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