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Summary of Significant Accounting Policies (Details)
customer in Thousands
 12 Months Ended
Dec. 31, 2019
USD ($)
registrant
customer
shares
Dec. 31, 2018
USD ($)
shares
Dec. 31, 2017
USD ($)
shares
Dec. 31, 2020
USD ($)
Jun. 30, 2020
USD ($)
Feb. 11, 2020
USD ($)
Mar. 31, 2019
USD ($)
Jan. 01, 2019
USD ($)
Accounting Policies [Line Items]                
Asset Retirement Obligation, Legally Restricted Assets, Fair Value $ 0              
Reinvested earnings in the business $ 345,964,000 $ 304,534,000           $ 0
Number of registrants filing combined report | registrant 2              
Operating Lease, Liability $ 13,588,000             8,000,000.0
Goodwill                
Goodwill arose from acquisition of subcontractor's business 1,100,000 1,100,000            
Related Party Transaction, Amounts of Transaction 4,700,000 4,200,000 $ 4,000,000.0          
Impairment of Long-Lived Assets                
Impairment of long-lived assets $ 0 0 $ 0          
Fair Value of Financial Instruments                
Short-term Debt, Terms P24M              
Notes Payable to Bank $ 205,000,000.0              
Notes payable to banks 5,000,000 0            
Debt Issuance Costs, Net 3,359,000 3,571,000            
Maximum borrowing capacity $ 225,000,000.0              
Nature of Operations:                
Number of customers served | customer 1,000,000,000              
Number of States in which Entity Operates 9              
Related Party Transactions                
Operating lease right-of-use assets $ 13,168,000 $ 0           $ 7,600,000
Property and Depreciation                
Stock Issued During Period, Shares, New Issues | shares 88,772 44,906 56,498          
Sales and Use Taxes:                
Non-income tax assessments accounted on a gross basis $ 20,042,000 $ 18,404,000 $ 17,905,000          
Long-term Debt [Member]                
Fair Value of Financial Instruments                
Notes Payable to Bank $ 205,000,000.0              
6.70% notes due 2019                
Fair Value of Financial Instruments                
Interest rate per annum (as a percent) 6.70% 6.70%            
Contracted services:                
Related Party Transactions                
Period of Fixed Price Contracts to Maintain Water Systems at Various Military Bases 50 years              
GSWC                
Accounting Policies [Line Items]                
Reinvested earnings in the business $ 257,434,000 $ 211,163,000            
Issuance of Common Shares   47,500,000            
Advances for Construction and Contributions in aid of Constructions                
Period for refund of advances for construction 40 years              
Alternative-Revenue Programs                
Number of months within which expected additional revenue collection is recorded 24 months              
Estimated useful lives of utility plant, as authorized by the CPUC                
Generation 40 years              
Fair Value of Financial Instruments                
Long-term debt-GSWC $ 376,467,000              
Debt Issuance Costs, Net 3,359,000 3,571,000            
Related Party Transactions                
Intercompany payable to Parent 158,845,000 0            
Inter-company payable to Parent 0 57,289,000            
Operating lease right-of-use assets $ 12,745,000 $ 0            
Property and Depreciation                
Aggregate composite rate for depreciation (as a percent) 2.20% 2.70% 2.60%          
Depreciation $ 31,700,000 $ 37,300,000 $ 36,500,000          
Depreciation on transportation equipment 316,000 238,000 242,000          
Sales and Use Taxes:                
Non-income tax assessments accounted on a gross basis 18,168,000 16,809,000 $ 16,402,000          
GSWC | Short-term Debt [Member]                
Fair Value of Financial Instruments                
Notes payable to banks 5,000,000.0              
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 284,699,000 324,978,000            
GSWC | Fair Value                
Fair Value of Financial Instruments                
Long-term debt-GSWC   $ 387,889,000            
GSWC | Fair Value | Level 2                
Fair Value of Financial Instruments                
Long-term debt-GSWC $ 376,467,000              
GSWC | Minimum                
Estimated useful lives of utility plant, as authorized by the CPUC                
Source of water supply 30 years              
Pumping 25 years              
Water treatment 20 years              
Transmission and distribution 25 years              
Other plant 7 years              
GSWC | Maximum                
Estimated useful lives of utility plant, as authorized by the CPUC                
Source of water supply 50 years              
Pumping 40 years              
Water treatment 35 years              
Transmission and distribution 55 years              
Other plant 40 years              
GSWC | 6.70% notes due 2019                
Fair Value of Financial Instruments                
Debt Instrument, Annual Principal Payment             $ 40,000,000  
Interest rate per annum (as a percent) 6.70% 6.70%         6.70%  
GSWC | Water:                
Nature of Operations:                
Number of customers served | customer 261              
GSWC | Electric:                
Nature of Operations:                
Number of customers served | customer 24              
American States Utility Services [Member] | Contracted services:                
Nature of Operations:                
Period of fixed price contracts to operate and maintain the water and/or wastewater systems at various military bases 50 years              
Sales [Member] | GSWC | Water:                
Nature of Operations:                
Percentage of total revenues 90.00%              
Sales [Member] | GSWC | Electric:                
Nature of Operations:                
Percentage of total revenues 90.00%              
AWR                
Fair Value of Financial Instruments                
Notes payable to banks $ 5,000,000 $ 0            
Maximum borrowing capacity $ 225,000,000.0              
AWR | Subsequent Event [Member]                
Fair Value of Financial Instruments                
Maximum borrowing capacity       $ 200,000,000.0 $ 200,000,000.0 $ 260,000,000.0    
AWR | Syndicated revolving credit facility                
Fair Value of Financial Instruments                
Maximum borrowing capacity             $ 200,000,000.0  
AWR | Syndicated revolving credit facility | Minimum                
Interest                
Line of Credit Facility, Current Borrowing Capacity             $ 150,000,000.0  
Common Stock [Member]                
Property and Depreciation                
Stock Issued During Period, Shares, New Issues | shares 89,000 77,000 110,000          
Common Stock [Member] | GSWC                
Property and Depreciation                
Stock Issued During Period, Shares, New Issues | shares   19