Summary of Significant Accounting Policies (Details)	12 Months Ended
	Dec. 31, 2025 USD ($) customer state unit registrant item shares	Dec. 31, 2024 USD ($) shares	Dec. 31, 2023 USD ($) shares
Nature of Operations:
Number of customers served | customer	1,000,000
Number of States in which Entity Operates | state	10
Property installed by developers and conveyed	$ 24,169,000	$ 9,840,000	$ 4,690,000
Number of registrants filing combined report | registrant	2
Related Party Transactions
Related Party Transaction, Amounts of Transaction	$ 5,200,000	5,300,000	$ 5,000,000.0
Issuance of Common Shares from an at-the-market program, net of issuance costs	$ 66,998,000	$ 88,661,000
Issuance of Common Shares from an at-the-market program, net of issuance costs (in Shares) | shares	27,674	25,196	18,371
Property and Depreciation
Depreciation on transportation equipment	$ 1,100,000	$ 600,000
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
GSWC
Nature of Operations:
Property installed by developers and conveyed	$ 24,169,000	9,840,000	4,690,000
Related Party Transactions
Issuance of Common Shares from an at-the-market program, net of issuance costs	$ 61,998,000	$ 40,000,000	$ 10,000,000
Issuance of Common Shares from an at-the-market program, net of issuance costs (in Shares) | shares	4.6250	2.7586	1
Property and Depreciation
Depreciation on transportation equipment			$ 200,000
Interest
Allowance for funds used during construction, additions	$ 600,000
BVES
Related Party Transactions
Related Party Transaction, Amounts of Transaction	3,500,000	$ 3,200,000	$ 3,500,000
Issuance of Common Shares from an at-the-market program, net of issuance costs	$ 6,000,000.0	$ 22,000,000.0
Issuance of Common Shares from an at-the-market program, net of issuance costs (in Shares) | shares	3.33	11
Interest
Allowance for funds used during construction, additions	$ 600,000	$ 1,300,000
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	$ 43,200,000	$ 39,800,000	$ 38,300,000
Depreciation on transportation equipment	$ 1,100,000	$ 600,000	$ 900,000
Water: | GSWC
Nature of Operations:
Number of customers served | customer	265,100
Water: | GSWC | San Juan Oak
Nature of Operations:
Public Utilities,Acquisition, Service Connections | unit	1,300
Property installed by developers and conveyed	$ 10,700,000
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: | Bay State Utility Service LLC ("BSUS")
Nature of Operations:
Period of Fixed Price Contracts to Maintain Water Systems at Various Military Bases	15 years