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Summary of Significant Accounting Policies (Details)
 12 Months Ended
Dec. 31, 2020
USD ($)
customer
registrant
shares
Dec. 31, 2019
USD ($)
shares
Dec. 31, 2018
USD ($)
shares
Nature of Operations:      
Number of customers served | customer 1,000,000    
Number of States in which Entity Operates 9    
Number of registrants filing combined report | registrant 2    
Related Party Transactions      
Related Party Transaction, Amounts of Transaction $ 4,900,000 $ 4,700,000 $ 4,200,000
Exercise of stock options and other issuance of Common Shares 30,000 519,000 546,000
Line of Credit Facility, Current Borrowing Capacity 200,000,000.0    
Regulatory Asset CEMA 4,400,000    
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      
Short-term Debt, Terms 24    
Common Stock [Member]      
Related Party Transactions      
Exercise of stock options and other issuance of Common Shares $ 30,000 $ 519,000 $ 546,000
Stock Issued During Period, Shares, New Issues | shares 42,000 89,000 77,000
GSWC      
Related Party Transactions      
Exercise of stock options and other issuance of Common Shares $ 60,000,000   $ 47,500,000
Due to Related Parties 0    
Regulatory Asset CEMA $ 4,100,000    
Property and Depreciation      
Aggregate composite rate for depreciation (as a percent) 2.20% 2.20% 2.70%
Depreciation $ 32,900,000 $ 31,700,000 $ 37,300,000
Depreciation on transportation equipment $ 353,000 316,000 238,000
Estimated useful lives of utility plant, as authorized by the CPUC      
Generation 40 years    
Fair Value of Financial Instruments      
Long-term debt-GSWC $ 559,752,000    
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 444,271,000 284,699,000  
GSWC | Fair Value      
Fair Value of Financial Instruments      
Long-term debt-GSWC   $ 376,467,000  
GSWC | Fair Value | Level 2      
Fair Value of Financial Instruments      
Long-term debt-GSWC $ 559,752,000    
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 | Common Stock [Member]      
Related Party Transactions      
Exercise of stock options and other issuance of Common Shares $ 60,000,000   $ 47,500,000
Stock Issued During Period, Shares, New Issues | shares 5 0 19
Bear Valley Electric Service, Inc.      
Related Party Transactions      
Related Party Transaction, Amounts of Transaction $ 1,300,000    
Regulatory Asset CEMA 295,000    
Interest      
Public Utilities, Allowance for Funds Used During Construction, Additions 200,000    
Parent [Member]      
Related Party Transactions      
Line of Credit Facility, Current Borrowing Capacity 30,000,000    
Notes Receivable, Related Parties 6,000,000    
Notes Payable, Related Parties $ 0    
Water: | GSWC      
Nature of Operations:      
Number of customers served | customer 261,800    
Electric: | Bear Valley Electric Service, Inc.      
Nature of Operations:      
Number of customers served | customer 24,500    
Contracted services: | American States Utility Services [Member]      
Nature of Operations:      
Period of fixed price contracts to operate and maintain the water and/or wastewater systems at various military bases 50 years