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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 |  |  |  |

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