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Summary of Significant Accounting Policies (Details) | 1 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2024 USD ($) shares | Dec. 31, 2024 USD ($) state item customer registrant shares | Dec. 31, 2023 USD ($) shares | Dec. 31, 2022 USD ($) shares | Jun. 05, 2024 USD ($) | Jan. 13, 2023 USD ($)
Nature of Operations: |  |  |  |  |  |
Number of customers served | customer |  | 1,000,000 |  |  |  |
Number of States in which Entity Operates | state |  | 10 |  |  |  |
Number of registrants filing combined report | registrant |  | 2 |  |  |  |
Related Party Transactions |  |  |  |  |  |
Related Party Transaction, Amounts of Transaction |  | $ 5,300,000 | $ 5,000,000.0 | $ 5,200,000 |  |
Issuance of Common Shares from an at-the-market program, net of issuance costs |  | $ 88,661,000 |  |  |  |
Issuance of Common Shares from an at-the-market program, net of issuance costs (in Shares) | shares |  | 25,196 | 18,371 | 25,956 |  |
Property and Depreciation |  |  |  |  |  |
Depreciation on transportation equipment |  | $ 600,000 | $ 900,000 | $ 400,000 |  |
Impairment of Long-Lived Assets |  |  |  |  |  |
Asset Retirement Obligation, Legally Restricted Assets, Fair Value |  | 0 |  |  |  |
Impairment of long-lived assets |  | 0 | 0 | $ 0 |  |
Goodwill |  |  |  |  |  |
Goodwill arose from acquisition of subcontractor's business |  | $ 1,100,000 | 1,100,000 |  |  |
Advances for Construction and Contributions in aid of Constructions |  |  |  |  |  |
Period for refund of advances for construction |  | 40 years |  |  |  |
Fair Value of Financial Instruments |  |  |  |  |  |
Long-term debt-GSWC |  | $ 608,184,000 |  |  |  |
Level 1 |  |  |  |  |  |
Fair Value of Financial Instruments |  |  |  |  |  |
Long-term debt-GSWC |  | 0 |  |  |  |
Level 3 |  |  |  |  |  |
Fair Value of Financial Instruments |  |  |  |  |  |
Long-term debt-GSWC |  | 0 |  |  |  |
Carrying Amount |  |  |  |  |  |
Fair Value of Financial Instruments |  |  |  |  |  |
Long-term debt-GSWC |  | 643,893,000 | 579,047,000 |  |  |
Fair Value | Level 2 |  |  |  |  |  |
Fair Value of Financial Instruments |  |  |  |  |  |
Long-term debt-GSWC |  | 608,184,000 | 556,214,000 |  |  |
GSWC |  |  |  |  |  |
Related Party Transactions |  |  |  |  |  |
Debt Instrument, Face Amount |  |  |  |  |  | $ 130,000,000.0
Issuance of Common Shares from an at-the-market program, net of issuance costs |  | $ 40,000,000.0 | $ 10,000,000 |  |  |
Issuance of Common Shares from an at-the-market program, net of issuance costs (in Shares) | shares |  | 2.7586 | 1 | 0 |  |
Property and Depreciation |  |  |  |  |  |
Depreciation on transportation equipment |  |  | $ 200,000 |  |  |
Estimated useful lives of utility plant, as authorized by the CPUC |  |  |  |  |  |
Generation |  | 40 years |  |  |  |
Fair Value of Financial Instruments |  |  |  |  |  |
Long-term debt-GSWC |  | $ 575,749,000 |  |  |  |
GSWC | Unsecured Private Placement Notes |  |  |  |  |  |
Related Party Transactions |  |  |  |  |  |
Debt Instrument, Face Amount |  |  |  |  | $ 65,000,000.0 | $ 130,000,000.0
GSWC | Level 1 |  |  |  |  |  |
Fair Value of Financial Instruments |  |  |  |  |  |
Long-term debt-GSWC |  | 0 |  |  |  |
GSWC | Level 3 |  |  |  |  |  |
Fair Value of Financial Instruments |  |  |  |  |  |
Long-term debt-GSWC |  | 0 |  |  |  |
GSWC | Carrying Amount |  |  |  |  |  |
Fair Value of Financial Instruments |  |  |  |  |  |
Long-term debt-GSWC |  | 608,893,000 | 544,047,000 |  |  |
GSWC | Fair Value | Level 2 |  |  |  |  |  |
Fair Value of Financial Instruments |  |  |  |  |  |
Long-term debt-GSWC |  | $ 575,749,000 | 522,883,000 |  |  |
GSWC | Minimum |  |  |  |  |  |
Estimated useful lives of utility plant, as authorized by the CPUC |  |  |  |  |  |
Source of water supply |  | 20 years |  |  |  |
Pumping |  | 26 years |  |  |  |
Water treatment |  | 26 years |  |  |  |
Transmission and Distribution |  | 15 years |  |  |  |
Other plant |  | 5 years |  |  |  |
GSWC | Maximum |  |  |  |  |  |
Estimated useful lives of utility plant, as authorized by the CPUC |  |  |  |  |  |
Source of water supply |  | 60 years |  |  |  |
Pumping |  | 41 years |  |  |  |
Water treatment |  | 32 years |  |  |  |
Transmission and Distribution |  | 80 years |  |  |  |
Other plant |  | 62 years |  |  |  |
BVES |  |  |  |  |  |
Related Party Transactions |  |  |  |  |  |
Related Party Transaction, Amounts of Transaction |  | $ 3,200,000 | 3,500,000 | $ 2,700,000 |  |
Issuance of Common Shares from an at-the-market program, net of issuance costs | $ 22,000,000.0 |  |  |  |  |
Issuance of Common Shares from an at-the-market program, net of issuance costs (in Shares) | shares | 11 |  |  |  |  |
Interest |  |  |  |  |  |
Public Utilities, Allowance for Funds Used During Construction, Additions |  | 1,300,000 |  | $ 100,000 |  |
BVES | Carrying Amount |  |  |  |  |  |
Fair Value of Financial Instruments |  |  |  |  |  |
Long-term debt-GSWC |  | $ 35,000,000 | $ 35,000,000 |  |  |
Bay State Utility Service LLC ("BSUS") |  |  |  |  |  |
Nature of Operations: |  |  |  |  |  |
Number of contracts | item |  | 1 |  |  |  |
Golden State Water Company and Bear Valley Electric Service, Inc. |  |  |  |  |  |
Property and Depreciation |  |  |  |  |  |
Aggregate composite rate for depreciation (as a percent) |  | 2.00% | 2.00% | 2.00% |  |
Depreciation |  | $ 39,800,000 | $ 38,300,000 | $ 37,300,000 |  |
Depreciation on transportation equipment |  | $ 600,000 | $ 900,000 |  |  |
Water: | GSWC |  |  |  |  |  |
Nature of Operations: |  |  |  |  |  |
Number of customers served | customer |  | 264,600 |  |  |  |
Electric: | BVES |  |  |  |  |  |
Nature of Operations: |  |  |  |  |  |
Number of customers served | customer |  | 24,900 |  |  |  |
Contracted services: | American States Utility Services | Minimum |  |  |  |  |  |
Nature of Operations: |  |  |  |  |  |
Period of fixed price contracts to operate and maintain the water and/or wastewater systems at various military bases |  | 15 years |  |  |  |
Contracted services: | American States Utility Services | Maximum |  |  |  |  |  |
Nature of Operations: |  |  |  |  |  |
Period of fixed price contracts to operate and maintain the water and/or wastewater systems at various military bases |  | 50 years |  |  |  |
Period of Fixed Price Contracts to Maintain Water Systems at Various Military Bases |  | 50 years |  |  |  |
Contracted services: | Patuxent River Utility Services LLC("PRUS") |  |  |  |  |  |
Nature of Operations: |  |  |  |  |  |
Period of fixed price contracts to operate and maintain the water and/or wastewater systems at various military bases |  | 50 years |  |  |  |
Initial price contract value and subject to annual economic price adjustments |  | $ 349,000,000 |  |  |  |
Revised price contract value after an inventory adjustment |  | 378,000,000 |  |  |  |
Contracted services: | Bay State Utility Service LLC ("BSUS") |  |  |  |  |  |
Nature of Operations: |  |  |  |  |  |
Initial price contract value and subject to annual economic price adjustments |  | $ 75,000,000.0 |  |  |  |
Period of Fixed Price Contracts to Maintain Water Systems at Various Military Bases |  | 15 years |  |  |  |
ContractInitialOrderValueToPerformAnEvaluationConstructionAndTransitionServices |  | $ 4,100,000 |  |  |  |

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