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Summary of Significant Accounting Policies: (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) customer registrant	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($)	Apr. 24, 2020 USD ($)	Dec. 31, 2019 USD ($)
Summary of significant accounting policies
Increase to provision	$ 393
Earnings reinvested in the business	348,742				$ 345,964
Operating lease right-of-use assets	$ 12,556				13,168
Nature of Operations:
Number of customers \| customer	1,000,000,000,000
Number of States in which Entity Operates	9
Basis of Presentation:
Number of registrants filing combined report \| registrant	2
Related Party Transactions
Notes payable to bank	$ 200,000				200,000
Notes payable to banks	$ 32,000				5,000
Related Party Transaction, Terms and Manner of Settlement	P24M
Syndicated Credit Facility
Related Party Transactions
Maximum borrowing capacity on line of credit	$ 232,000
GOLDEN STATE WATER COMPANY
Summary of significant accounting policies
Increase to provision	393
Earnings reinvested in the business	257,340				257,434
Operating lease right-of-use assets	12,235				12,745
Related Party Transactions
Payments to affiliate for corporate office administrative and general costs	1,400	$ 1,100
Intercompany payable to Parent	$ 182,447				$ 158,845
GOLDEN STATE WATER COMPANY \| Water Service Utility Operations
Nature of Operations:
Number of customers \| customer	261,000
GOLDEN STATE WATER COMPANY \| Electric Service Utility Operations
Nature of Operations:
Number of customers \| customer	24,000
ASUS \| Contracted Services
Nature of Operations:
Period of fixed price contracts to operate and maintain water systems at various military bases	50 years
Sales \| GOLDEN STATE WATER COMPANY \| Water Service Utility Operations
Nature of Operations:
Concentration Risk, Percentage	90.00%
Sales \| GOLDEN STATE WATER COMPANY \| Electric Service Utility Operations
Nature of Operations:
Concentration Risk, Percentage	90.00%
Subsequent Event [Member] \| AWR \| Syndicated Credit Facility
Related Party Transactions
Line of Credit Facility, Incremental Expansion of Borrowing Capacity			$ 35,000
Subsequent Event [Member] \| Electric Service Utility Operations \| Syndicated Credit Facility
Related Party Transactions
Line of Credit Facility, Current Borrowing Capacity				$ 50,000
Subsequent Event [Member] \| Minimum [Member] \| AWR \| Syndicated Credit Facility
Related Party Transactions
Line of Credit Facility, Current Borrowing Capacity			200,000
Subsequent Event [Member] \| Maximum [Member] \| AWR \| Syndicated Credit Facility
Related Party Transactions
Line of Credit Facility, Current Borrowing Capacity			$ 260,000