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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 |  |  |  |  |  |

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