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Summary of Significant Accounting Policies (Regulatory Rate Filings) (Details) |  |  |  |  |  |  |  |  |  |  |  |  | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Feb. 01, 2020 USD ($) | Jan. 31, 2020 USD ($) | Jan. 20, 2020 USD ($) | Dec. 20, 2019 USD ($) | Nov. 20, 2019 USD ($) | Oct. 01, 2019 USD ($) | Sep. 30, 2019 USD ($) | Aug. 30, 2019 USD ($) | Feb. 01, 2019 USD ($) | Jan. 28, 2019 USD ($) | Jan. 01, 2019 USD ($) | Sep. 14, 2018 USD ($) | Dec. 31, 2019 USD ($) mi² | Mar. 31, 2019 | Dec. 31, 2019 USD ($) mi² | Dec. 31, 2019 USD ($) mi² | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Feb. 07, 2020 | Jan. 28, 2020 | Dec. 06, 2019 USD ($) | Oct. 18, 2019 USD ($) | Aug. 01, 2019 USD ($) | Jul. 26, 2019 USD ($) | Feb. 08, 2019 USD ($) | Jan. 04, 2018 USD ($)
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Balancing and Memorandum Accounts, Net Under-Collected |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,428,000 |  | $ 1,428,000 | $ 1,428,000 |  |  |  |  |  |  |  |  |  |
Balancing and Memorandum Cost Recovery Account, Refunds (Collections) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (2,603,000) | $ 1,000 |  |  |  |  |  |  |  |  |
Balancing and Memorandum Revenue Account, Increase (Decrease) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (4,984,000) | 3,827,000 |  |  |  |  |  |  |  |  |
Revenue from contracts with customers |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 447,720,000 | 389,302,000 | $ 381,777,000 |  |  |  |  |  |  |  |
Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Service area in square miles | mi² |  |  |  |  |  |  |  |  |  |  |  |  | 246 |  | 246 | 246 |  |  |  |  |  |  |  |  |  |
CPED Proposed Penalty, Per Offense, Per Day |  |  |  |  |  |  |  |  |  |  |  | $ 500 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
CPED Proposed Penalty, Per Offense, Per Day |  |  |  |  |  |  |  |  |  |  |  | 50,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
2018 WCMA [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Balancing and Memorandum Revenue Account, Increase (Decrease) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ (9,386,000) | $ 9,386,000 |  |  |  |  |  |  |  |  |
San Jose Water Company [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
CPED Calculated Double-Billing Refund |  |  |  |  |  |  |  |  |  |  |  | 4,935,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
San Jose Water Company [Member] | General Rate Case No. 18-01-004 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested Rate Increase, Year One |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 34,288,000
Requested Rate Increase, Year One, Percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 9.76%
Requested Rate Increase, Year Two |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 14,232,000
Requested Rate Increase, Year Two, Percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3.70%
Requested Rate Increase, Year Three |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 20,582,000
Requested Rate Increase, Year Three, Percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5.17%
Requested Balancing and Memorandum Accounts, net |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 20,725,000
San Jose Water Company [Member] | Advice Letter No. 528/528A [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Approved Rate Increase (Decrease), Amount |  |  |  |  |  |  |  |  |  |  | $ 16,378,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Approved Rate Increase (Decrease), Percentage |  |  |  |  |  |  |  |  |  |  | 4.55% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Balancing and Memorandum Cost Recovery Account, Refunds (Collections) |  |  |  |  |  |  |  |  |  |  | $ 27,045,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
San Jose Water Company [Member] | Advice Letter No. 530 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved Service Charge Refund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,020,000 |
San Jose Water Company [Member] | CPED Calculated Refund 2014-2016 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
CPED Calculated Service Charge Refund |  |  |  |  |  |  |  |  |  |  |  | 2,061,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
San Jose Water Company [Member] | CPED Calculated Refund 1987-2013 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Customer Credit Refund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,100,000 |  |
San Jose Water Company [Member] | Advice Letter No. 531 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Approved Reimbursement Fee, Percentage |  |  |  |  |  |  |  |  |  |  |  |  |  | 1.40% | 1.23% |  |  |  |  |  |  |  |  |  |  |
San Jose Water Company [Member] | Advice Letter No. 532 [Member] | 2018 WCMA [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Balancing and Memorandum Revenue Account, Increase (Decrease) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 9,386,000 |  |  |  |  |  |  |  |  |  |
San Jose Water Company [Member] | Advice Letter No. 532 [Member] | 2019 WCMA [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Balancing and Memorandum Revenue Account, Increase (Decrease) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ (639,000) |  |  |  |  |  |  |  |  |  |
San Jose Water Company [Member] | Advice Letter No. 537 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Memorandum Account, Requested Recovery (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] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested Rate Increase, Year One |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,300,000 |  |  |  |  |
Requested Rate Increase, Year One, Percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0.61% |  |  |  |  |
Requested Rate Increase, Year Two |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 4,000,000.0 |  |  |  |  |
Requested Rate Increase, Year Two, Percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1.04% |  |  |  |  |
Requested Rate Increase, Year Three |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,500,000 |  |  |  |  |
Requested Rate Increase, Year Three, Percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0.65% |  |  |  |  |
Regulatory Rate Filings, Requested Rate Increase, Year Four |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 300,000 |  |  |  |  |
Regulatory Rate Filings, Requested Rate Increase, Percent of Authorized Revenue, Year Four |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0.09% |  |  |  |  |
San Jose Water Company [Member] | Advice Letter No. 534 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Memorandum Account, Requested Recovery (Refund) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,243,000 |  |  |
San Jose Water Company [Member] | Advice Letter No. 535 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Amount |  |  |  |  |  |  |  | $ 700,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Percentage |  |  |  |  |  |  |  | 0.17% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Connecticut Water Company [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Authorized WICA Surcharge, Percentage |  |  |  |  |  |  |  |  |  |  |  |  | 3.24% |  | 3.24% | 3.24% |  |  |  |  |  |  |  |  |  |
Connecticut Water Company [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested WICA Surcharge, Percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5.75% | 5.84% |  |  |  |  |  |
Requested WICA Surcharge Increase (Decrease), Percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (0.09%) | 2.60% |  |  |  |  |  |
Connecticut Water Company & Heritage Valley Water Company [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Authorized WRA Revenue |  |  |  |  |  |  |  |  |  |  |  |  | $ 18,588,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Revenue from contracts with customers |  |  |  |  |  |  |  |  |  |  |  |  | 20,428,000 |  |  |  |  |  |  |  |  |  |  |  |  |
WRA Adjustment to Revenue |  |  |  |  |  |  |  |  |  |  |  |  | $ (1,840,000) |  |  |  |  |  |  |  |  |  |  |  |  |
Avon Water Company [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Authorized WICA Surcharge, Percentage |  |  |  |  |  |  |  |  |  |  |  |  | 9.31% |  | 9.31% | 9.31% |  |  |  |  |  |  |  |  |  |
SJWTX, Inc. [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Authorized Regulatory Surcharge, Per Thousand Gallons |  |  |  |  |  |  |  |  | $ 1.05 | $ 1.13 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deer Creek Ranch [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Authorized Regulatory Surcharge, Per Thousand Gallons |  |  |  |  |  | $ 2.02 | $ 2.19 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Authorized Regulatory Base Charge, Per Thousand Gallons |  |  |  |  |  | $ 3.04 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Canyon Lake [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Authorized Regulatory Surcharge, Per Thousand Gallons | $ 0.95 | $ 1.05 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maine Water Company [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Amount |  |  |  | $ 221,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Percentage |  |  |  | 14.70% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maine Water Company [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Amount |  |  | $ 371 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maine Water Company [Member] | Minimum [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested WISC Surcharge Increase (Decrease), Percentage |  |  | 1.76% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maine Water Company [Member] | Maximum [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested WISC Surcharge Increase (Decrease), Percentage |  |  | 3.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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