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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