Summary of Significant Accounting Policies (Details) customer in Thousands		1 Months Ended	12 Months Ended
	Sep. 29, 2017	Sep. 30, 2017	Dec. 31, 2017 USD ($) registrant customer	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Oct. 31, 2015 USD ($)	Oct. 27, 2015 USD ($)
Summary of significant accounting policies
Debt Issuance Costs, Net			$ 3,902,000	$ 4,271,000
Related Party Transaction, Amounts of Transaction			$ 4,000,000	3,900,000	$ 2,600,000
Basis of Presentation:
Number of registrants filing combined report | registrant			2
Impairment of Long-Lived Assets
Impairment of long-lived assets			$ 0	0	0
Goodwill
Goodwill arose from acquisition of subcontractor's business			1,100,000	1,100,000
Sales and Use Taxes:
Non-income tax assessments accounted on a gross basis			$ 17,905,000	16,801,000	$ 16,636,000
Military Utility Privatization Subsidiaries
Summary of significant accounting policies
Period of Fixed Price Contracts to Maintain Water Systems at Various Military Bases			50 years
GSWC
Summary of significant accounting policies
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense			$ 1,011,000
Debt Issuance Costs, Net			$ 3,902,000	$ 4,271,000
Property and Depreciation
Aggregate composite rate for depreciation (as a percent)			2.60%	2.90%	3.20%
Depreciation on transportation equipment			$ 242,000	$ 259,000	$ 641,000
Estimated useful lives of utility plant, as authorized by the CPUC
Generation			40 years
Interest
Amount recorded in AFUDC			$ 0	101,000	694,000
Alternative-Revenue Programs
Number of months within which expected additional revenue collection is recorded			24 months
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			$ 424,042,000
Sales and Use Taxes:
Franchise fees billed to customers and recorded as operating revenue			3,600,000	3,500,000	3,800,000
Non-income tax assessments accounted on a gross basis			16,402,000	15,444,000	15,244,000
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			325,265,000	325,582,000
GSWC | Fair Value
Fair Value of Financial Instruments
Long-term debt-GSWC				423,124,000
GSWC | Fair Value | Level 2
Fair Value of Financial Instruments
Long-term debt-GSWC			$ 424,042,000
GSWC | Minimum [Member]
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 | Notes Payable, Other Payables [Member]
Related Party Transactions
Debt Instrument, Face Amount						$ 40,000,000	$ 40,000,000
Debt Instrument, Maximum Borrowing Capacity						40,000,000	40,000,000
GSWC | Notes Payable, Other Payables [Member] | Notes Receivable
Related Party Transactions
Notes Receivable, Related Parties, Current			$ 0	0
GSWC | Purchase, production, distribution and sale of water
Nature of Operations:
Number of customers served | customer			259
Interest
Weighted cost of capital (as a percent)			8.34%
GSWC | Electric
Nature of Operations:
Number of customers served | customer			24
Sales and Use Taxes:
Cost Of Debt Percent			6.96%
American States Utility Services [Member]
Sales and Use Taxes:
Non-income tax assessments accounted on a gross basis			$ 287,000	$ 309,000	$ 367,000
American States Utility Services [Member] | Notes Payable, Other Payables [Member]
Related Party Transactions
Debt Instrument, Face Amount						10,000,000	10,000,000
Debt Instrument, Maximum Borrowing Capacity						$ 10,000,000	$ 10,000,000
American States Utility Services [Member] | Military Utility Privatization Subsidiaries
Nature of Operations:
Period of fixed price contracts to operate and maintain the water and/or wastewater systems at various military bases		50 years	50 years
Fort Riley [Member] | Minimum [Member]
Nature of Operations:
Period of Transition to Operate and Maintain Water Systems at New Military Base	6 months	6 months
Fort Riley [Member] | Maximum
Nature of Operations:
Period of Transition to Operate and Maintain Water Systems at New Military Base	12 months	12 months
Consolidated Entities [Member]
Summary of significant accounting policies
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense			$ 1,042,000
Sales [Member] | GSWC | Purchase, production, distribution and sale of water
Nature of Operations:
Percentage of total revenues			90.00%	90.00%	90.00%
Syndicated revolving credit facility | Syndicated revolving credit facility
Related Party Transactions
Maximum borrowing capacity on line of credit			$ 150,000,000
AWR
Related Party Transactions
Maximum borrowing capacity on line of credit			150,000,000
AWR | Syndicated revolving credit facility | Syndicated revolving credit facility
Related Party Transactions
Maximum borrowing capacity on line of credit			$ 150,000,000