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Summary of Significant Accounting Policies: (Details) |  | 3 Months Ended | 9 Months Ended |  |  |  |
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Jun. 28, 2023 USD ($) | Sep. 30, 2023 USD ($) state | Mar. 31, 2023 USD ($) shares | Sep. 30, 2022 USD ($) | Sep. 30, 2023 USD ($) customer state registrant shares | Sep. 30, 2022 USD ($) shares | Nov. 06, 2023 USD ($) | Jun. 16, 2023 USD ($) | Jan. 13, 2023 USD ($) | Dec. 31, 2022 USD ($)
Summary of significant accounting policies |  |  |  |  |  |  |  |  |  |
Number of customers | customer |  |  |  |  | 1,000,000 |  |  |  |  |
Number of states in which entity operates | state |  | 10 |  |  | 10 |  |  |  |  |
Number of registrants filing combined report | registrant |  |  |  |  | 2 |  |  |  |  |
Issuances of Common Shares under stock-based compensation plans (in shares) | shares |  |  |  |  | 18,351 | 24,612 |  |  |  |
Debt instrument term | 5 years |  |  |  |  |  |  |  |  |
Incremental expansion of borrowing capacity |  | $ 75,000,000 |  |  | $ 75,000,000 |  |  |  |  |
Notes payable to banks |  | 236,500,000 |  |  | 236,500,000 |  |  |  |  | $ 22,000,000
Notes payable to banks |  | 37,000,000 |  |  | 37,000,000 |  |  |  |  | 255,500,000
Syndicated Revolving Credit Facility member [Member] |  |  |  |  |  |  |  |  |  |
Summary of significant accounting policies |  |  |  |  |  |  |  |  |  |
Line of credit arrangements |  | 566,000 |  |  | 566,000 |  |  |  |  |
Repayments of Lines of Credit | $ 280,000,000 |  |  |  |  |  |  |  |  |
Letter of Credit | Subsequent Event |  |  |  |  |  |  |  |  |  |
Summary of significant accounting policies |  |  |  |  |  |  |  |  |  |
Maximum borrowing capacity on line of credit |  |  |  |  |  |  | $ 10,000,000 |  |  |
Syndicated Revolving Credit Facility member [Member] |  |  |  |  |  |  |  |  |  |
Summary of significant accounting policies |  |  |  |  |  |  |  |  |  |
Notes payable to banks |  | 133,500,000 |  |  | 133,500,000 |  |  |  |  |
ASUS |  |  |  |  |  |  |  |  |  |
Summary of significant accounting policies |  |  |  |  |  |  |  |  |  |
Payments to affiliate for corporate office administrative and general costs |  | 1,100,000 |  | $ 1,300,000 | $ 3,800,000 | $ 4,000,000 |  |  |  |
ASUS | Contracted services: |  |  |  |  |  |  |  |  |  |
Summary of significant accounting policies |  |  |  |  |  |  |  |  |  |
Period of fixed price contracts to operate and maintain water systems at various military bases |  |  |  |  | 50 years |  |  |  |  |
BVES |  |  |  |  |  |  |  |  |  |
Summary of significant accounting policies |  |  |  |  |  |  |  |  |  |
Payments to affiliate for corporate office administrative and general costs |  | 707,000 |  | $ 663,000 | $ 2,800,000 | $ 2,100,000 |  |  |  |
Current borrowing capacity |  |  |  |  |  |  |  | $ 35,000,000 |  |
Incremental expansion of borrowing capacity |  |  |  |  |  |  |  | 25,000,000 |  |
Maximum borrowing capacity on line of credit |  |  |  |  |  |  |  | $ 50,000,000 |  |
BVES | Syndicated Revolving Credit Facility member [Member] |  |  |  |  |  |  |  |  |  |
Summary of significant accounting policies |  |  |  |  |  |  |  |  |  |
Notes payable to banks |  | 37,000,000 |  |  | $ 37,000,000 |  |  |  |  |
BVES | Electric: |  |  |  |  |  |  |  |  |  |
Summary of significant accounting policies |  |  |  |  |  |  |  |  |  |
Number of customers | customer |  |  |  |  | 24,700 |  |  |  |  |
GSWC |  |  |  |  |  |  |  |  |  |
Summary of significant accounting policies |  |  |  |  |  |  |  |  |  |
Issuances of Common Shares under stock-based compensation plans (in shares) | shares |  |  | 1 |  | 1 |  |  |  |  |
Issuance of common share to parent |  |  | $ 10,000,000 |  | $ 10,000,000 |  |  |  |  |
Intercompany borrowings payment term |  |  |  |  | 24 months |  |  |  |  |
Notes payable to banks |  | 0 |  |  | $ 0 |  |  |  |  | $ 129,000,000
GSWC | Syndicated Revolving Credit Facility member [Member] |  |  |  |  |  |  |  |  |  |
Summary of significant accounting policies |  |  |  |  |  |  |  |  |  |
Line of credit arrangements |  | 802,000 |  |  | 802,000 |  |  |  |  |
GSWC | Letter of Credit | Subsequent Event |  |  |  |  |  |  |  |  |  |
Summary of significant accounting policies |  |  |  |  |  |  |  |  |  |
Maximum borrowing capacity on line of credit |  |  |  |  |  |  | 20,000,000 |  |  |
GSWC | Syndicated Revolving Credit Facility member [Member] |  |  |  |  |  |  |  |  |  |
Summary of significant accounting policies |  |  |  |  |  |  |  |  |  |
Notes payable to banks |  | 103,000,000 |  |  | $ 103,000,000 |  |  |  |  |
GSWC | Unsecured Private Placement Notes |  |  |  |  |  |  |  |  |  |
Summary of significant accounting policies |  |  |  |  |  |  |  |  |  |
Face amount of debt |  |  |  |  |  |  |  |  | $ 130,000,000 |
GSWC | Water: |  |  |  |  |  |  |  |  |  |
Summary of significant accounting policies |  |  |  |  |  |  |  |  |  |
Number of customers | customer |  |  |  |  | 264,000 |  |  |  |  |
Patuxent River Utility Services LLC ( PRUS) | Contracted services: |  |  |  |  |  |  |  |  |  |
Summary of significant accounting policies |  |  |  |  |  |  |  |  |  |
Period of fixed price contracts to operate and maintain water systems at various military bases |  |  |  |  | 50 years |  |  |  |  |
Initial price contract value |  | $ 349,000,000 |  |  |  |  |  |  |  |
Period of transition to operate and maintain water system at new military base |  | 6 months |  |  |  |  |  |  |  |
Bay State Utility Service LLC (" BSUS") | Contracted services: |  |  |  |  |  |  |  |  |  |
Summary of significant accounting policies |  |  |  |  |  |  |  |  |  |
Period of fixed price contracts to operate and maintain water systems at various military bases |  |  |  |  | 15 years |  |  |  |  |
Initial price contract value |  | $ 45,000,000 |  |  |  |  |  |  |  |
Contract initial order value to perform an evaluation, construction, and transition services. |  |  |  |  | $ 2,300,000 |  |  |  |  |
Maximum | Syndicated Revolving Credit Facility member [Member] |  |  |  |  |  |  |  |  |  |
Summary of significant accounting policies |  |  |  |  |  |  |  |  |  |
Current borrowing capacity |  | $ 150,000,000 |  |  | $ 150,000,000 |  |  |  |  |
Ratio of Indebtedness to Net Capital |  | 0.65 |  |  | 0.65 |  |  |  |  |
Maximum | Syndicated Revolving Credit Facility member [Member] | Subsequent Event |  |  |  |  |  |  |  |  |  |
Summary of significant accounting policies |  |  |  |  |  |  |  |  |  |
Current borrowing capacity |  |  |  |  |  |  | $ 165,000,000 |  |  |
Maximum | ASUS |  |  |  |  |  |  |  |  |  |
Summary of significant accounting policies |  |  |  |  |  |  |  |  |  |
Period of fixed price contracts to operate and maintain water systems at various military bases |  |  |  |  | 50 years |  |  |  |  |
Maximum | GSWC | Syndicated Revolving Credit Facility member [Member] |  |  |  |  |  |  |  |  |  |
Summary of significant accounting policies |  |  |  |  |  |  |  |  |  |
Current borrowing capacity |  | $ 200,000,000 |  |  | $ 200,000,000 |  |  |  |  |

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