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Long-Term Debt - Narrative (Details) - USD ($) |  |  |  |  |  |  | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Dec. 15, 2020 | Nov. 02, 2020 | Aug. 11, 2020 | Mar. 12, 2020 | Oct. 08, 2019 | Mar. 28, 2019 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 23, 2020 | Dec. 19, 2019
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Repayments of Long-term Debt |  |  |  |  |  |  | $ 28,931,000 | $ 1,400,000 | $ 0 |  |
Long-term debt |  |  |  |  |  |  | 1,352,320,000 | 1,292,586,000 |  |  |
Significant Observable Inputs (Level 2) |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Fair value of long-term debt |  |  |  |  |  |  | 1,570,727,000 | 1,396,205,000 |  |  |
SJW Group | Senior notes |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Minimum net worth required for compliance |  |  |  |  |  |  | $ 175,000,000 |  |  |  |
Cumulative net income, percentage included in net worth for compliance |  |  |  |  |  |  | 30.00% |  |  |  |
Principal Debt Sold |  |  | $ 50,000,000 |  |  |  |  |  |  |  |
Stated interest rates |  |  | 2.47% |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  |  |  | $ 610,000,000 | 560,000,000 |  |  |
SJW Group | Senior notes | Minimum |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stated interest rates |  |  |  |  |  |  | 2.47% |  |  |  |
SJW Group | Senior notes | Financial Guarantee |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Ratio of indebtedness to total capital |  |  |  |  |  |  | 0.6666 |  |  |  |
SJW Group | Senior notes | 2011 Senior Notes |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Ratio of indebtedness to total capital |  |  |  |  |  |  | 0.6666 |  |  |  |
SJW Group | Senior notes | 2019 and 2020 Unsecured Notes |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Ratio of indebtedness to total capital |  |  |  |  |  |  | 0.70 |  |  |  |
SJW Group | Senior notes | Series 2019A |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Principal Debt Sold |  |  |  |  | $ 310,000,000 |  |  |  |  |  |
Stated interest rates |  |  |  |  | 3.05% |  |  |  |  |  |
SJW Group | Senior notes | Series 2019B |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Principal Debt Sold |  |  |  |  | $ 75,000,000 |  |  |  |  |  |
Stated interest rates |  |  |  |  | 3.15% |  |  |  |  |  |
SJW Group | Senior notes | Series 2019C |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Principal Debt Sold |  |  |  |  | $ 125,000,000 |  |  |  |  |  |
Stated interest rates |  |  |  |  | 3.53% |  |  |  |  |  |
San Jose Water Company | California Pollution Control Financing Authority Revenue Bonds |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  |  |  | $ 440,000,000 | 450,000,000 |  |  |
San Jose Water Company | Senior notes |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |
Restricted payment, maximum payment amount |  |  |  |  |  |  | $ 20,000,000 |  |  |  |
Long-term debt |  |  |  |  |  |  | $ 320,000,000 | 330,000,000 |  |  |
San Jose Water Company | Senior notes | Minimum |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stated interest rates |  |  |  |  |  |  | 4.29% |  |  |  |
San Jose Water Company | Senior notes | Series C |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Repayments of Long-term Debt |  | $ 10,000,000 |  |  |  |  |  |  |  |  |
San Jose Water Company | Senior notes | Series M |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Principal Debt Sold |  |  |  |  |  | $ 80,000,000 |  |  |  |  |
Stated interest rates |  |  |  |  |  | 4.29% |  |  |  |  |
San Jose Water Company | Public utility, bonds | California Pollution Control Financing Authority Revenue Bonds |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  |  |  | $ 120,000,000 | 120,000,000 |  |  |
San Jose Water Company | Public utility, bonds | California Pollution Control Financing Authority Revenue Bonds | Minimum |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stated interest rates |  |  |  |  |  |  | 4.75% |  |  |  |
Connecticut Water Service, Inc. | Bank term loans |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Ratio of indebtedness to total capital |  |  |  |  |  |  | 0.60 |  |  |  |
Long-term debt |  |  |  |  |  |  | $ 22,169,000 | 23,935,000 |  |  |
Connecticut Water Service, Inc. | Bank term loans | Minimum |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stated interest rates |  |  |  |  |  |  | 4.09% |  |  |  |
Connecticut Water Company |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  |  |  | $ 225,400,000 | 198,646,000 |  |  |
Connecticut Water Company | Senior notes |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Ratio of indebtedness to total capital |  |  |  |  |  |  | 0.60 |  |  |  |
Principal Debt Sold |  |  |  | $ 35,000,000 |  |  |  |  |  |  |
Stated interest rates |  |  |  | 3.51% |  |  |  |  |  |  |
Long-term debt |  |  |  |  |  |  | $ 70,000,000 | 35,000,000 |  |  |
Interest coverage ratio |  |  |  |  |  |  | 3 |  |  |  |
Connecticut Water Company | Senior notes | Minimum |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stated interest rates |  |  |  |  |  |  | 3.51% |  |  |  |
Connecticut Water Company | Bank term loans |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Ratio of indebtedness to total capital |  |  |  |  |  |  | 0.60 |  |  |  |
Repayments of Long-term Debt | $ 8,000,000 |  |  |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  |  |  | $ 111,090,000 | 119,090,000 |  |  |
Interest coverage ratio |  |  |  |  |  |  | 3 |  |  |  |
Connecticut Water Company | Bank term loans | Minimum |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stated interest rates |  |  |  |  |  |  | 3.51% |  |  |  |
Connecticut Water Company | Public utility, bonds | Water Facilities Revenue Bonds |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  |  |  | $ 44,310,000 |  |  |  |
SJWTX, Inc. | Senior notes |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Ratio of indebtedness to total capital |  |  |  |  |  |  | 0.6666 |  |  |  |
Stated interest rates |  |  |  |  |  |  | 6.27% |  |  |  |
Ratio of net income available for interest charges for the trailing 12-month calendar period to interest charges |  |  |  |  |  |  | 1.75 |  |  |  |
Long-term debt |  |  |  |  |  |  | $ 15,000,000 | 15,000,000 |  |  |
SJWTX, Inc. | Senior notes | Financial Guarantee |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Minimum net worth required for compliance |  |  |  |  |  |  | $ 125,000,000 |  |  |  |
SJWTX, Inc. | Senior notes | Financial Guarantee | Minimum |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Cumulative net income, percentage included in net worth for compliance |  |  |  |  |  |  | 30.00% |  |  |  |
Maine Water Company |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  |  |  | $ 39,751,000 | 38,032,000 |  |  |
Maine Water Company | Bank term loans |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Ratio of indebtedness to total capital |  |  |  |  |  |  | 0.60 |  |  |  |
Long-term debt |  |  |  |  |  |  | $ 17,500,000 | 17,500,000 |  |  |
Interest coverage ratio |  |  |  |  |  |  | 3 |  |  |  |
Maine Water Company | Bank term loans | Minimum |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stated interest rates |  |  |  |  |  |  | 4.18% |  |  |  |
Maine Water Company | Public utility, bonds | State revolving fund loans |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  |  |  | $ 18,651,000 | 16,032,000 |  |  |
Maine Water Company | Public utility, bonds | State revolving fund loans | Minimum |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stated interest rates |  |  |  |  |  |  | 0.00% |  |  |  |
Maine Water Company | Public utility, bonds | Other First Mortgage Bonds |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Ratio of indebtedness to total capital |  |  |  |  |  |  | 0.65 |  |  |  |
Stated interest rates |  |  |  |  |  |  | 8.95% |  |  |  |
Long-term debt |  |  |  |  |  |  | $ 3,600,000 | $ 4,500,000 |  |  |
Interest coverage ratio |  |  |  |  |  |  | 3 |  |  |  |
Dividend restriction, cumulative dividends paid maximum |  |  |  |  |  |  | $ 120,000 |  |  |  |
Maine Water Company | Public utility, bonds | State Revolving Fund Loans Series T |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stated interest rates |  |  |  |  |  |  |  |  |  | 1.00% |
Long-term debt |  |  |  |  |  |  |  |  |  | $ 4,000,000 |
Maine Water Company | Public utility, bonds | State Revolving Fund Loans Series S |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stated interest rates |  |  |  |  |  |  |  |  |  |  | 1.00%
Long-term debt |  |  |  |  |  |  |  |  |  |  | $ 5,000,000

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